Fixes support for generators as well as manifest and install tasks when working in a Rails engine.

[pbhogan]

Addresses #138

• app:stimulus:* tasks correctly run in test/dummy and put files in the right places.
• stimulus:* tasks exist and put files under the engine namespace.
• The stimulus controller generator, when run in the engine root, includes the engine namespace.
• Namespaces are correctly handled in index.js and in terms of where files are placed.

Note: a small extension of Rake is included to make defining the top-level tasks possible.

[wakproductions]

fwiw I was also working on a Rails engine and encountered the same problem. In my case, the problem manifested as this error:

$ rails app:javascript:install:esbuild
Unrecognized command "app:template" (Rails::Command::UnrecognizedCommandError)
Did you mean?  app:tmp:create

The task Stimulus was looking to run was namespaced as app:app:template because of Railties. The same problem also affects other rake tasks such as jsbundling-rails and turbo-rails. I hope this fix gets merged so that we can use the automated generators more easily. Lmk if there's anything I can do to assist.