Suggestion: Document Testing Facilities

[bjthehun]

Langium offers testing facilities for languages (i.e. check for parser errors, test that validation found an issue, etc.). While these are highly appreciated, I only stumbled on those by accident. I would have liked to found them in the documentation.

Perhaps the MiniLogo Tutorial could be extended to check for typical syntax errors, test that duplicate defs and params are found, etc.

[Lotes]

I think the new learning section in the documentation covers some parts of testing opportunities now.
What is still open are the LSP features and other not so well-known parts of Langium (I know parsing, linking, validation and generation). I suggest to add a testing section for each of the upcoming recipes.

@bjthehun WDYT? Is the new learning section covering what you have requested here?