Use the vendored parser to drive autocomplete.

[cristianoc]

The vendored parser has a recovery mechanism where parsing continues past an error. This makes it suitable to support autocomplete, which is triggered on incomplete code which would normally not parse.
This replaces the previous hand-crafted mini-parser used specifically for autocomplete, which would start from the cursor position, and try to parse backwards figuring out the context the cursor happens to be in.

The new mechanism starts from the current position and the parse tree produced by the parser. For example, List. gets parsed as List._ where the dummy identifier _ is inserted by the recovery mechanism.
If the cursor position is immediately after ., the mechanism finds the relevant node in the parse tree which represents expression List._, and uses that information for autocomplete.

One of the benefits of this new approach is that relying on the actual parser gives a better handling of corner cases (e.g. support for template literals inside strings comes for free), and avoids the maintenance burden of supporting/extending the ad-hoc parser.
Another benefit is that the parser provides contextual information, which can be used to decide whether e.g. the current context is a value context or type context, or variant context, or field name context. So that let x = List.m will complete values from List, while let f : Js.Promise. will complete types from module Js.Promise.