Add ruby.intellisense configuration option

[castwide]

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This PR adds the ruby.intellisense configuration to allow method selection for go to/peek definition similar to ruby.codeCompletion. Valid options are solargraph, rubyLocate, and none.

[wingrunr21]

Should this PR also remove the extension dependency on solargraph?

[castwide]

The extension dependency is still necessary for the solargraph options to work. This PR helps ensure that only one intellisense provider is registered, so "go to definition" doesn't return duplicated entries.

[wingrunr21]

Gotcha. I guess my concern is if users select rubyLocate or none, they are going to wonder why the solargraph extension is a requirement if they are not using it.

[castwide]

Fair point. If neither ruby.codeCompletion nor ruby.intellisense is set to solargraph, the extension is not strictly required (although it will still provide access to documentation through the command palette). Making it a dependency is just the easiest way to integrate it.

[wingrunr21]

Ok. I'm going to open an issue on the topic (as I'm not convinced having it as a hard dependency is the right choice) but I also don't want to hold up this change. A standalone issue will allow other commentary and we can always remove the dependency later.