Support built-in LSP server

[koic]

This PR has two parts committed. That is, the import LSP server from Standard and support RuboCop built-in LSP. Please keep these commits.

Part 1: Import LSP server from Standard

Preparing to resolve #11686

This commit imports built-in LSP server of https://github.com/standardrb/standard into RuboCop core.

The standard gem's original license (MIT) is written at the beginning of the imported files.
@searls @will

Part 2: Support built-in LSP

Resolves #11686

This commit supports built-in language server.

The integrated language server feature is described in the docs/modules/ROOT/pages/usage/lsp.adoc.

LSP option

% bundle exec rubocop --help
(snip)

LSP Option:
        --lsp                        Start a language server listening on STDIN.

The language server --lsp is disabled by default because it is not yet
stable as an integrated feature.

Run rubocop --lsp command from LSP client.

When the language server is started, the command displays the language server's PID:

$ ps aux | grep rubocop
user             17414   0.0  0.2  5557716 144376   ??  Ss    4:48PM   0:02.13 /Users/user/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/bin/rubocop --lsp

The language server supports textDocument/formatting method and is autocorrectable. The autocorrection is safe.

Design note

Same concept as standardrb/standard#475. See there for details.

Usage Example (Eglot)

I'm running the LSP on Emacs with Eglot. It behaves roughly as expected.
https://github.com/joaotavora/eglot

Add the following to your Emacs configuration file (e.g. ~/.emacs.d/init.el):

(require 'eglot)

(add-to-list 'eglot-server-programs '(ruby-mode . ("bundle" "exec" "rubocop" "--lsp")))
(add-hook 'ruby-mode-hook 'eglot-ensure)

Below is an example of additional setting for autocorrecting on save:

(add-hook 'ruby-mode-hook (lambda () (add-hook 'before-save-hook 'eglot-format-buffer nil 'local)))

In theory it should be available for other LSP clients as well.

NOTE: I've been using this language server locally with Emacs (Eglot) for a while and it's performing well.

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[bbatsov]

Impressive work! I'll need a bit of time to review this more carefully, but at a glance it looks pretty good!

I'm running the LSP on Emacs with Eglot. It behaves roughly as expected.

I didn't know you were an Emacs user as well! M-x forever! 🚀

[bbatsov]

You may want to mention that we'll consider this experimental feature for a while.

[koic]

Sure! I updated it.

[bbatsov]

I'm wondering if a name more consistent with the existing commands (e..g --start-lsp-server) is not idea as well. We can keep --lsp as a shorter alias.

[koic]

I'm concerned that LSP client is the trigger that reconnects or shuts down the LSP server. For Elgot, use M-x elgot-reconnection or M-x elgot-shutdown to reconnect or shutdown. In other words, the rubocop command that the user recognizes in the LSP setting may be enough with --lsp option.
How about just providing --lsp option to start with? So --start-lsp-server will be added later when really needed for user.

[bbatsov]

Got it. If that's the only flag the users will ever need we can stick with it.

[bbatsov]

I think in this section we should also mention which LSP features does our implementation support, so it's clearer for the end users. We might also mention the advantages of preferring LSP for editor integration over --rubocop-server and just shelling out a RuboCop process.

This might encourage more people to try out the LSP support.

[koic]

I have added this text to the doc in response to the review. What do you think?

[bbatsov]

Overall the code looks good to me, plus I guess it's already battle tested to some extent by Standard.rb.

[koic]

@bbatsov I'm developing vscode-rubocop:
https://github.com/koic/vscode-rubocop

It is based on vscode-standard-ruby. (Thank you @searls and @kddnewton. Mentioned in LICENSE.)
https://github.com/standardrb/vscode-standard-ruby

After RuboCop 1.53.0 is released, I plan to transfer it to github.com/rubocop/vscode-rubocop and publish it to the Visual Studio Marketplace.