Support completion LSP feature

[msujew]

Adds support for Langium-style code completion - it's a bit more structurally sound compared to Langium:

1. Since we cannot purely rely on runtime reflection for certain features (such as synthetic AST nodes, if a reference appears as its first feature), this change adds a completion_gen file that helps us with some of the reflection handling. In Langium, we were able to do this inline.
2. Implements ATN simulation for the completion use case. This is a sounder approach than what we did in Langium, where we had to interpret the current grammar at runtime.
3. Generates a new completion parser as well. We need to track a lot of ATN intermediate information for the completion. While we theoretically could track this during the normal parse as well, this would slow down the normal parser, which is undesirable.
4. Based on top of the error recovery work, otherwise most of these features wouldn't work at all (since completion operates on incomplete input).
5. Adds 2 ways to extend the functionality of the completion provider:
6. A CompletionContributor works similar to the common override of the CompletionProvider in Langium - specify exactly how a completion item is generated from a specific source token type and ATN state. This also works for cross references and snippets/templates.
7. A CompletionFilter that allows more fine-grained control over which SymbolDescription elements are returned as part of the cross-reference completion logic. Acts similar to the existing LanguageScopeProvider.

This is quite an extensive change, but it's also extensively tested. Also fixes a bunch of bugs in the error recovery, mostly related to nil handling at EOF.

[spoenemann]

Works great!

[spoenemann]

This is a great starting point. We can refine it over time where needed.