Unify and improve code paths for Workspace Symbol and Document Symbol

[DavisVaughan]

Connected to posit-dev/positron#3822

See #571 and in particular #571 (comment)

• We use the indexer for workspace symbols, but not for document symbols. We should try and share more infrastructure here. Note that the indexer currently produces a flat list of symbols, but for outlines it is quite useful to have a nested list of symbols instead.
• We should produce nested "sections", particularly in the Document Symbol response, where it shows up in the Outline view. This would allow ## second ---- to be a child of # first ----, and would also nest any functions / variables inside these section headers too.