Add support for documentation comments

[BinderDavid]

Part of the solution to #325

[jiribenes]

@b-studios, I took a look at how to continue, and the biggest problem I could see is that when we'll need two different annotations.
One for Tree -> DocComment in parser, and one for Symbol -> DocComment which gets populated in Namer, and then used in the LSP (on hover -> resolve to symbol -> retrieve the doc comment from symbol).

Is there a better way? We could also store the DocComment on the AST node of a declaration, but I remember you were somewhat opposed to this. Or should we somehow keep only the Tree -> DocComment annotation, and then when hovering, do a "resolve to symbol -> resolve to tree -> retrieve the doc comment from tree"?

WDYT?

---

object Annotations {
  // ... existing annotations ...

  type DocComments = Vector[String]

  /**
   * Documentation comments for symbols such as functions, types, effects etc.
   * Stores multiple doc comments in order of appearance.
   */
  val Documentation = Annotation[symbols.Symbol, DocComments](
    "Documentation",
    "documentation comments associated with"
  )

[b-studios]

Will we only have doc comments on things that are symbols? If yes, then we can always go from tree to symbol and only store Symbol -> DocComment, no?

ah... there is a hole in my reasoning. This will only be possible after Namer...

[b-studios]

Maybe easiest to store it in the tree, but I have to see how this looks and whether this is too annoying when doing source to source rewritings.