Remove RuboCop dependency and fallback to SyntaxTree formatting

[vinistock]

Motivation

Closes #76
Building on top of #183

If RuboCop is not present in the current application, we can fallback to formatting using SyntaxTree. Additionally, this allows us to drop the dependency on RuboCop and simply try to require it when available.

Implementation

Both the diagnostics and formatting runners will now exit early without defining the classes if RuboCop is not present. If the runner classes don't exist, then we can exit early in diagnostics and fallback to SyntaxTree on formatting.

Automated Tests

Honestly, I'm not sure about the best way to test this. If you have ideas on how to fake that RuboCop is not present in the bundle, please let me know.

Manual Tests

It's a bit of work, but here are the steps if you're interested in testing it out.

With RuboCop

1. On the Ruby LSP project itself (which uses RuboCop), start the server
2. Edit some files
3. Verify that RuboCop diagnostics are showing up
4. Verify that formatting the file fixes the RuboCop violations

Without RuboCop

1. Create a minimal project structure (Gemfile, dev.yml and an example.rb file)
2. Configure dev.yml with the desired Ruby version and dev up
3. In the Gemfile, add the Ruby LSP pointing to this branch (e.g.: gem "ruby-lsp", path: "../ruby-lsp")
4. Start the plugin
5. Edit example.rb
6. Verify no diagnostics show up
7. Verify that saving the file still formats (this time using SyntaxTree)

[paracycle]

We can test this by modifying the $LOAD_PATH like so:

$ irb
> require "rubocop"
true

$ irb
> $LOAD_PATH.delete_if {|path| path.start_with?(Gem.loaded_specs["rubocop"].full_gem_path) }
> require "rubocop"
LoadError: cannot load such file -- rubocop

(of course, we would need to recover the original $LOAD_PATH afterwards)

[paracycle]

🎉

[vinistock]

Renamed the method and also made unloading constants a bit more resilient. It was making the tests flaky when trying to remove constants, so I just wrapped it in a rescue.