Explore exporting a Rails engine to provide an API for the LSP

[vinistock]

If we expose a Rails engine from the Ruby LSP, we can add APIs for applications that opt-in to mounting the engine and use it from the Ruby LSP if the server is running.

From the engine, we can expose any sort of development information interesting to surface in the editor. Some ideas:

• Auto-reload when saving files
• Displaying column information for models
• Highlighting Active Record queries that do not use an index
• Jump from a controller action to its corresponding route (think like go to definition)

We can also take advantage that the application is already running to execute things in-process. For example, we can expose an action to execute a rake task. Invoking the rake task from an already loaded application is bound to be faster than starting a fresh Ruby process from the CLI.

Concerns

If the user is debugging code, it has the potential to get the LSP stuck.

[vinistock]

Will be addressed in #587.