Add support for Code Actions under Ruby LSP

[searls]

In order to support code actions (e.g. "Quick Fix" in VS Code), we need a more sophisticated way of invoking RuboCop than our kinda hacky stdin / stdout capture technique that invokes the CLI version. Ruby LSP provides a great way for doing this: extend the base runner and rely on instance variables of the runner to get at the actual RuboCop objects, then translate that to LSP types from the language_server_protocol gem's interfaces.

Lots of thoughts:

1. This is an unacceptable and unsustainable amount of duplication from Ruby LSP, as @koic and I have both pointed out as being a risk, and a reason to figure out a consolidation strategy in the medium/long term
2. I tried to maintain compatibility with the Standardizer object (which the Ruby LSP addon and the built-in server both depend on) is now returning diagnostic objects, whereas the built-in standard --lsp server is expecting JSON parsed from stdin. That needs to be translated appropriately before shipping anything Done ✅
3. I'm inclined to freeze/deprecate the built-in --lsp server in favor of pointing people to Ruby LSP and/or delegating to the RuboCop LSP
4. I'm similarly inclined to gut the remaining Standard-specific code that went into this Ruby LSP add-on by either delegating to RuboCop or by seeing the add-on itself move into RuboCop in such a way as to be compatible with both Standard and RuboCop
5. This already actually works under VS Code, which is neat