Support full LSP spec

[msujew]

In order for downstream users to easily create implementation of lesser used LSP functionalities (such as monikers, inlay hints, or document colors), we should at least have interfaces (and ideally some abstract class) that cover this functionality.

This is a collection issue for all other issues that are trying to track specific functionalities of the LSP spec.

• [x] https://github.com/langium/langium/issues/594
• [ ] https://github.com/langium/langium/issues/556
• [x] https://github.com/langium/langium/issues/596
• [x] https://github.com/langium/langium/issues/597
• [x] https://github.com/langium/langium/issues/598
• [x] https://github.com/langium/langium/issues/599

[bnhmn]

I found that Langium has an interface DocumentSymbolProvider but not something equivalent for Workspace symbols. Workspace Symbols can be quite handy because they allow to quickly navigate to symbols across documents.

It works for me like this but it would be nice to have an official interface for it.