feat: add rubocop support

[glepnir]

Fix #2699

[gongfarmer]

Thank you!

The solution implemented did not quite work for me, some of us rubyists are not using bundler.

It works if I configure it to omit the bundle command:

require'lspconfig'.rubocop.setup{
  cmd = { 'rubocop', '--lsp' }
}

[glepnir]

oh . I just see this in your linked pr . it's emacs elgot start server command . so I just copy this if not work could you make a pr for it ?

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

[gongfarmer]

#2706