Support Rails auto-detection for engines/railties

I have checked that this feature is not already implemented

• This feature does not exist

Use case

The LSP currently does not auto-detect Rails for Engine/Railtie projects, presumably as of #2218. The ability to use the Rails add-on when working on such projects would be extremely useful, particularly for tests and feature congruence when working on Rails applications and common dependencies simultaneously.

Description

A few options come to mind:

1. The Rails LSP add-on to be automatically installed upon detection of a Railtie-based project, instead of just a concrete application
2. Automatic inclusion upon detection of a Rails dummy application
3. Manual configuration via VS Code Settings to "tell" Ruby LSP to install the add-on

Implementation

Ideally the auto-detection for Rails apps would be modified to add an additional check - whether or not the main class descends from Rails::Railtie. Variable-depth namespacing may add complexity to finding the main class, but I'm no expert. Applying the current check a second time at [spec|test]/*/config/application.rb may be a suitable alternative for basing inclusion on the existence of a dummy app - something included as standard by the Rails plugin generator.

If these options aren't feasible, could some mechanism be added to allow users to manually specify that the Rails add-on should be included?

Start by reviewing the existing Rails application auto-detection referenced in #2218 and the checks around [spec|test]/*/config/application.rb. Compare detection for concrete applications, Railtie-based projects, and dummy applications, then determine which supported configuration should cause the Rails add-on to be included. Done means the chosen detection or manual configuration works for the stated project types with corresponding tests.