Handle deferred route drawing

[andyw8]

In rails/rails#51614, @gmcgibbon has improved boot time by deferring route drawing. This will likely ship in the next release of Rails, so will require a small change for us.

For ruby-lsp-rails we could either:

• Defer loading until the first definition for a route (faster)
• Run Rails.application.routes_reloader.execute_unless_loaded (if defined) as part of server.rb to eagerly load the routes (slower).