Add basic completion for self fields and locals

[julienduchesne]

Closes #3

[github-actions]

Benchstat (compared to main):

name                                                                    old time/op  new time/op  delta
Definition/test_goto_definition_for_var_myvar-2                         6.14µs ± 1%  6.01µs ± 0%  -2.23%  (p=0.029 n=4+4)
Definition/test_goto_definition_on_function_helper-2                    6.24µs ± 1%  6.14µs ± 2%  -1.62%  (p=0.032 n=5+5)
Definition/test_goto_inner_definition-2                                 6.38µs ± 3%  6.24µs ± 1%  -2.26%  (p=0.008 n=5+5)
Definition/test_goto_super_index-2                                      6.51µs ± 2%  6.32µs ± 1%  -2.94%  (p=0.008 n=5+5)
Definition/test_goto_super_nested-2                                     6.59µs ± 1%  6.36µs ± 1%  -3.46%  (p=0.008 n=5+5)
Definition/test_goto_self_object_field_function-2                       6.54µs ± 2%  6.40µs ± 0%  -2.18%  (p=0.008 n=5+5)
Definition/test_goto_super_object_field_local_defined_obj_'foo'-2       6.88µs ± 1%  6.82µs ± 1%  -0.77%  (p=0.048 n=5+5)
Definition/test_goto_super_object_field_local_defined_obj_'g'-2         6.98µs ± 1%  7.06µs ± 2%    ~     (p=0.310 n=5+5)
Definition/test_goto_local_var_from_other_local_var-2                   5.59µs ± 1%  5.50µs ± 1%  -1.56%  (p=0.016 n=5+5)
Definition/test_goto_local_obj_field_from_'self.attr'_from_other_obj-2  8.08µs ± 1%  8.00µs ± 1%  -0.99%  (p=0.032 n=5+5)
Definition/test_goto_local_object_'obj'_via_obj_index_'obj.foo'-2       6.52µs ± 1%  6.49µs ± 1%    ~     (p=0.421 n=5+5)
Definition/test_goto_imported_file-2                                    16.6µs ± 1%  16.8µs ± 1%  +1.11%  (p=0.032 n=5+5)
Definition/test_goto_imported_file_at_lhs_index-2                       6.87µs ± 1%  6.85µs ± 1%    ~     (p=0.841 n=5+5)
Definition/test_goto_imported_file_at_rhs_index-2                       6.93µs ± 1%  6.79µs ± 1%  -1.98%  (p=0.008 n=5+5)
Definition/goto_import_index-2                                          6.27µs ± 1%  6.27µs ± 1%    ~     (p=0.952 n=5+5)
Definition/goto_attribute_of_nested_import-2                            7.76µs ± 1%  7.74µs ± 1%    ~     (p=0.421 n=5+5)
Definition/goto_dollar_attribute-2                                      6.92µs ± 1%  6.76µs ± 0%  -2.27%  (p=0.008 n=5+5)
Definition/goto_dollar_sub_attribute-2                                  7.42µs ± 0%  7.36µs ± 1%    ~     (p=0.056 n=5+5)
Definition/goto_dollar_doesn't_follow_to_imports-2                      6.89µs ± 1%  6.82µs ± 0%  -0.98%  (p=0.008 n=5+5)
Definition/goto_attribute_of_nested_import_no_object_intermediary-2     6.95µs ± 0%  6.93µs ± 0%    ~     (p=0.095 n=5+5)
Definition/goto_self_in_import_in_binary-2                              37.9µs ± 1%  38.5µs ± 1%  +1.60%  (p=0.016 n=5+5)
Definition/goto_self_attribute_from_local-2                             6.50µs ± 0%  6.48µs ± 1%    ~     (p=0.310 n=5+5)
Definition/goto_function_parameter_from_inside_function-2               6.63µs ± 1%  6.62µs ± 2%    ~     (p=0.841 n=5+5)
Definition/goto_local_func_param-2                                      6.91µs ± 2%  6.87µs ± 1%    ~     (p=0.690 n=5+5)
Definition/goto_self_complex_scope_1-2                                  7.15µs ± 1%  7.10µs ± 1%    ~     (p=0.095 n=5+5)
Definition/goto_self_complex_scope_2-2                                  7.79µs ± 1%  7.73µs ± 1%    ~     (p=0.087 n=5+5)
Definition/goto_with_overrides:_clobber_string-2                        90.6µs ± 1%  91.6µs ± 1%  +1.09%  (p=0.008 n=5+5)
Definition/goto_with_overrides:_clobber_nested_string-2                 93.0µs ± 1%  95.1µs ± 1%  +2.18%  (p=0.008 n=5+5)
Definition/goto_with_overrides:_clobber_map-2                           90.8µs ± 1%  92.2µs ± 2%  +1.56%  (p=0.016 n=5+5)
Definition/goto_with_overrides:_map_(multiple_definitions)-2            93.9µs ± 2%  95.2µs ± 1%    ~     (p=0.286 n=4+5)
Definition/goto_with_overrides:_nested_map_(multiple_definitions)-2      100µs ± 1%   102µs ± 1%  +1.77%  (p=0.008 n=5+5)
Definition/goto_with_overrides:_string_carried_from_super-2             90.4µs ± 1%  91.1µs ± 1%  +0.78%  (p=0.032 n=5+5)
Definition/goto_with_overrides:_nested_string_carried_from_super-2      94.7µs ± 1%  92.8µs ± 0%  -1.93%  (p=0.029 n=4+4)
Definition/goto_with_overrides:_string_carried_from_local-2             93.2µs ± 0%  92.9µs ± 1%    ~     (p=0.222 n=5+5)
Definition/goto_with_overrides:_string_carried_from_import-2            92.9µs ± 1%  93.2µs ± 1%    ~     (p=0.548 n=5+5)
Definition/goto_with_overrides:_string_carried_from_second_import-2     93.5µs ± 1%  93.1µs ± 1%    ~     (p=0.310 n=5+5)
Definition/goto_deeply_nested_imported_attribute-2                      10.5µs ± 1%  10.4µs ± 1%  -0.83%  (p=0.016 n=5+5)
Definition/goto_deeply_nested_imported_attribute_through_self-2         44.2µs ± 0%  43.9µs ± 0%  -0.56%  (p=0.016 n=5+4)
Definition/goto_computed_field_name_object_field-2                      6.69µs ± 1%  6.63µs ± 0%  -0.93%  (p=0.008 n=5+5)
Definition/goto_field_through_function-2                                7.05µs ± 1%  6.93µs ± 0%  -1.76%  (p=0.008 n=5+5)
Definition/goto_field_through_function-created_object-2                  128µs ± 0%   129µs ± 1%  +0.60%  (p=0.032 n=5+5)
Definition/goto_field_through_builder_pattern_attribute-2               10.9µs ± 1%  10.8µs ± 0%  -0.75%  (p=0.008 n=5+5)
Definition/goto_field_through_mixin_attribute-2                         8.97µs ± 0%  8.98µs ± 2%    ~     (p=0.730 n=5+5)