Scala3doc: cook comments

[abgruszecki]

Resolve @define-s and expand @inheritdoc-s.

[abgruszecki]

There are still problems with some definitions in the standard library. The issue is that we don't correctly attach the comment to some trees. I'll fix this in a separate PR.

[abgruszecki]

See https://scala3doc.virtuslab.com/pr-10506/scala3-stdlib/-scala%203/-a-p-i/scala/collection/immutable/-list$.html vs https://scala3doc.virtuslab.com/pr-master/scala3-stdlib/-scala%203/-a-p-i/scala/collection/immutable/-list$.html .

[abgruszecki]

All issues with attaching comments are now fixed, all missing variable warnings when documenting stdlib are genuine.

[abgruszecki]

@odersky I needed to modify the scanner to attach docstrings in some cases where it not attach them before. Can you quickly review 60a8b72 (and only that commit)?

[romanowski]

I was a bit merge-happy here, sorry. I can revert this merge or apply fixes if changes in parser needs them.
cc @odersky