For multimethods, add implementation and dispatch key to document-symbols

[jwhitlark]

Is your feature request related to a problem? Please describe.
Multimethod implementations don't show up in Calva outline. Calva uses clojure-lsp to populate the outline view, per @PEZ.

Describe the solution you'd like
I'd like to see both the multifn name and dispatch value show up in the outline. Even just adding the multifn names would be an improvement, if dispatch values required additional or upstream work.

Describe alternatives you've considered
In theory, I could declare a dispatch value, but it would require code changes only for this reason, and is a non-starter on the projects I work on.

Additional context
Discussion of this particular issue started on #662 with @mainej. Note that I am not certain document-symbols is the correct location, but seems highly likely, after conversations.

[mainej]

A few implementation notes:

1. Currently document-symbols filters the analysis for :var-definitions only. This would mean including :var-usages that have :defmethod true.
2. The dispatch-val is a little tricky. @borkdude are you interested in including dispatch-val in the analysis of a defmethod? If not, clojure-lsp would need to use the rewrite-clj tree. The most efficient way to search through that tree is with clojure-lsp.parser/to-pos, which we could call using position data from the clj-kondo analysis element. That would still mean multiple calls to parser/to-pos, so we'd have to keep an eye on performance. Alternatively, as @jwhitlark said, we could omit the dispatch-val and only show the multifn name.
3. Whatever changes we decide on should probably be applied to workspace-symbols too.

[borkdude]

@mainej I think that's potentially interesting, also for detecting duplicate dispatch values which we could warn about. There's already an issue about that here. The dispatch val is always at the same position right? So then the analysis should not be so hard to implement. Feel free to open an issue and a candidate PR.

[mainej]

@borkdude I'll open an issue. I can't commit to a PR right now. @ericdallo I'm also just helping spec out this problem, not claiming it. It sounds like a quick fix would be to split this into two parts:

1. add the multifn name to document-symbols now, without the dispatch-val, and
2. add the dispatch-val after it's available from clj-kondo

[ericdallo]

Agreed @mainej !