Add comment provider

[Lotes]

• serializes to $comment property

• comes with default multiline comments

• replaces consumption of comments in documentation provider by a comment provider call

• one thing that bothers me is that comments can get duplicated (I guess)

A ::= B c
B ::= a b

For file:

/** comment */
a b c

• because then $comment(A) == $comment(B) == '/** comment */'
• @msujew what do you think? Can we live with this or should we filter out the comment for upper nodes when a lower node already has the comment?

[spoenemann]

@Lotes could you describe the main use cases for this, besides the documentation provider?

[msujew]

@spoenemann There are two relevant design changes in here:

1. Adopters of Langium can choose to override the getComment method to change how other services derive the comment from a given AST node. Right now, all services use findCommentNode, which isn't overridable.
2. Since we lose the CST during serialization, we also lose all semantically relevant comments. You could argue that comments should never be semantically relevant, but this is not the case for Langium. For example, what if I want to generate interface definitions similar to TSDoc from a serialized version of my AST? We need the AST comment for that, which is exactly what the serializer change in this PR accomplishes.

[msujew]

what do you think? Can we live with this or should we filter out the comment for upper nodes when a lower node already has the comment?

We can live with that, since filtering it out might be potentially destructive.

[spoenemann]

Ok, makes sense.

[msujew]

Looks good to me, thanks 👍

One minor comment, see below.