Add definitionStyle option to control multi-head goto definition

[flowerett]

Solves this issue

When a function has multiple heads/clauses, editors like Zed show a picker UI instead of jumping directly.

The new "definitionStyle" initializationOption ("all" or "first") lets users choose whether to return all definition sites or just the first one.

the new initializationOption will look like:

"dexter": {
      "initialization_options": {
        ...
        "definitionStyle": "first" # or "all"
      }
}

[flowerett]

@JesseHerrick would be great to know your thoughts about this approach and naming.

[JesseHerrick]

Hey @flowerett, thanks for the PR! This same thing happens with all editors and LSPs when there are multiple function heads for the same definition with the same arity. IMO that is a feature. While it's something we could change in the LSP, this is something I'd actually leave up to the LSP client and editor to configure to their liking. Editors should be able to just say "give me the first option" instead of opening the list if they want it. In NeoVim this isn't too tricky to configure, but maybe in Zed it's a pain. Thoughts?