Allow rubyfmt to be part of the "auto" formatter option

I have checked that this feature is not already implemented

• This feature does not exist

Use case

Setting formatter: "auto" should check for add ons as well, if "ruby-lsp-rubyfmt" is available it should allow to fallback to rubyfmt instead of setting formatter to "none".

This is helpful in scenarios when we have to deal with multiple codebases with multiple code linting tools.

Description

I work with multiple codebases using different linters, rubyfmt, rubocop, etc. I can't set my formatter to "rubocop" or "auto", because I need "rubyfmt" for some.

The only workaround for me is to constantly change my formatter everytime I work on a different codebase.

It seems, "auto" only supports "rubocop" and "syntax trees" as these are Lsp Supported.

I propose to allow add-ons to be part of "auto" if it exists in the workspace.

Implementation

I did a quick tour of the codebase and I'm assuming while setting global state for the formatter we need to detect add-ons formatters too:

    def detect_formatter(direct_dependencies, all_dependencies)
      # NOTE: Intentionally no $ at end, since we want to match rubocop-shopify, etc.
      return "rubocop_internal" if direct_dependencies.any?(/^rubocop/)

      syntax_tree_is_direct_dependency = direct_dependencies.include?("syntax_tree")
      return "syntax_tree" if syntax_tree_is_direct_dependency

      rubocop_is_transitive_dependency = all_dependencies.include?("rubocop")
      return "rubocop_internal" if dot_rubocop_yml_present && rubocop_is_transitive_dependency

      "none"
    end

If it's accepted I can add the logic to detect add on dependencies

Start by locating the detect_formatter entry point shown in the issue and inspect how direct and transitive dependencies determine the auto formatter. Confirm the existing rubocop and syntax_tree behavior, then verify that an available ruby-lsp-rubyfmt add-on makes auto select rubyfmt instead of none without changing other cases.