[WIP] Add semantic highlighting modifiers based on symbol declaration location

[Krzysztof-Cieslak]

Requires bunch of more work which I'm not sure I'm bothered enough to do:

• [ ] Write actual classifier rather than proof of concept I've done (TODO in DeclarationLocationClassifier.fs file)
• [ ] Disable feature by default (TODO in LspHelpers.fs file)
• [ ] Support setting multiple declaration locations qualifiers at the same time (something may be external and FSharpCore at the same time, we can send both modifiers to the client so users have control over how they want it displayed)

Example usage - symbols coming from FSharpCore are underlined: