Don’t use Bundler if bundle exec fails

[ParadoxV5]

If bundle exec fails – typically because of missing Gemfile/Gemfile.lock, but can also because of [P.S. Steep not in Gemfile or even] missing or unreachable bundle executable – the LSP shall fall back to try whichever steep executable is available – typically the one natually from gem install steep.

I believe I can similate this by overriding the steep executable choice to a small launcher script in the plugin configs.

Together with soutaro/steep#968 or an equivalent feature, this allows Steep to work out-of-the-box as an LSP with no further setup required.
(This lifts the Bundler requirement; that lifts the requirement of Steepfile in the workspace root.)
Though of course users are encouraged to set up a Gemfile and a Steepfile.