C++: Improve parameter highlighting

This adds parameter highlighting for reference parameters and defaulted
parameters. For example:

```cpp
auto strip_prefix_only(std::string& s,
                       Hidden_Homonym skip_hidden_homonym = {}) const
    -> Affixing_Result<Prefix>;
```

Previously both parameters were only highlighted as variables.

runtime/queries/cpp/highlights.scm

@@ -49,6 +49,13 @@
 (this) @variable.builtin
 (nullptr) @constant.builtin
 
+; Parameters
+
+(parameter_declaration
+  declarator: (reference_declarator (identifier) @variable.parameter))
+(optional_parameter_declaration
+  declarator: (identifier) @variable.parameter)
+
 ; Keywords
 
 (template_argument_list (["<" ">"] @punctuation.bracket))