Add support for LSP completion using the surrounding context

[ebresafegaga]

This pull request adds support for auto-completion based on the context (from a record, type, or contract.)

Case 1

Say we have the following:

{
[.]
} | SchemaFromNix

With this change, if the contract SchemaFromNix, represents a record or dictionary contract, we will get completion corresponding to its fields.

Case 2

This will for work not only for basic records, but also for nested records:

let Tf = import "./contract.ncl" in
{
  config = {
    data.dhall.my_dhall_resource = {
      [.]
    }
  }
} | Tf.Config

Here, we would get a completion based on the fields of Tf.Config.config.data.dhall.my_dhall_resource, if it is a record dictionary contract.

Case 3

This also includes support for nested record completion based on the context contract.

let Tf = import "./contract.ncl" in
{
  config[.]
} | Tf.Config

In this case, we would get completion based on the fields of Tf.Config.config, if it is a record or dictionary contract.

Additional improvements

1. Adds error recovery if parsing record fields fails

2. Adds support for completion when we have a dictionary contract: say we have the following:

let foo | {_: {a: Str, b: Str}} = <record definition> in 
foo[.] 

This doesn't get a completion, becuase the dictionary has dynamic fields, but when we do this:

foo.random_dynamic_field[.]

We will get a completion

3. Adds support for contracts represented by variables or static record access (See #998)

---

Closes #995, Closes #998

[yannham]

LGTM. Nice trick to get the parent node by just looking on the left, giving a logarithmic time algo at worst.