Improve Rails app detection

[andyw8]

Motivation

Closes https://github.com/Shopify/ruby-lsp/issues/1466

Implementation

Check for the presence of bin/rails as suggested by Rafael.

Automated Tests

Included

Manual Tests

It's slightly awkward as we need to ensure the setup process doesn't end up installing the published version:

• Check out this branch.
• Temporarily change the version to something nonsense value, e.g. 0.14.999 so you can easily identify it.
• Run gem build
• Run gem install <filename>
• Check out the rails/rails repo.
• Delete the current .ruby-lsp directory if it exists
• Reload the window
• Ensure the Ruby LSP output window doesn't show a failure for bin/rails
• Ensure .ruby-lsp/Gemfile doesn't contain ruby-lsp-rails
• Remember to uninstall the temporary local gem afterwards, or it may result in confusing failures.

[andyw8]

There is some corresponding discussion happening in https://github.com/Shopify/ruby-lsp-rails/pull/284 about what is the right approach for this.

[vinistock]

My two cents is that, as long as the ruby-lsp-rails is not failing, we can delay this decision a little bit because we may find value in having the addon present for Ruby apps that have rails in the Gemfile, but aren't Rails applications.

[andyw8]

Handled by https://github.com/Shopify/ruby-lsp-rails/pull/284