Completions

[wkillerud]

Sets up the server capability and a handler for completion requests.

I've run into an issue with how strict the current parser is:
https://github.com/sass/dart-sass-language-server/pull/34/files#diff-dfd87bf70431e0c34c7d859b41e81359474d0cff8488684c8796738aed0c3e72R87-R120

.a {
  display: /* [1] */ ;
}

/* [1] Here the parser throws (Error: Expected expression).
   Since we don't have a Stylesheet we can't tell what the context/AstNode is at this position.
   We would like to know that we're in a Declaration with the name "display", with no children,
   so we can provide completion items for the allowed values for the display property.

   In this particular case we could probably work around it by looking at the raw string,
   but strings have their own limitations and problems (how to handle scope for Sass variables
   for instance). */

It makes perfect sense for a compiler to be this strict, but (especially for this feature) the language server needs the parser to run in a mode where it does its best to recover from these errors and returns the Stylesheet in whatever state it currently is, errors and all.