Add option for generating RBIs for Rails engines

[andyw8]

(Paired with @vinistock)

Motivation

Closes #1182

Implementation

An --app_root option was added to the tapioca dsl command. This can be used to specify the path to the dummy app for an engine.

There were several places in the code which assumed the app is always at the root of the project. These were updated to take the app_root option into consideration.

Tests

The test approach is styled after the existing RBI generation tests. We create files for the app as normal, but then move the necessary files into the test/dummy path before running the dsl commmand.

[KaanOzkan]

I'd prefer writing the files to /test/dummy directly on line 347 but not a blocker.

[andyw8]

Agreed, unfortunately the top-level before(:all) sets the project path so we would need to restructure the tests first.

[KaanOzkan]

I'm a bit confused. I was only referring to the lib files. More specifically I was thinking of changing the 2 writes to:
@project.write("test/dummy/lib/post.rb", <<~RB) & @project.write("test/dummy/lib/comment.rb", <<~RB)

and removing the line FileUtils.mv(@project.path + "/lib", engine_path).

We don't have to do it, I'm just trying to understand the project path blocker 🙂

[andyw8]

Ah, I understand now. I was thinking of config/environment.rb which gets created in the before(:all). I've opened #1225 to address this.