Make template idempotent

[tvararu]

By making the template idempotent, it makes it possible to iterate and continue adding optional features to the template, with downstream users being able to benefit from these improvements by re-running it against their project.

This change makes it possible to run the template against an existing project. It will skip over any already applied changes:

$ rails new --skip-bundle --skip-jbuilder --skip-test --skip-action-text --skip-action-mailer --skip-action-mailbox \
  -m rails-template/template.rb test-project
$ cd test-project
$ bin/rails app:template LOCATION=../rails-template/template.rb
         run  bundle install from "."
   identical  Procfile.dev
   identical  bin/dev
         run  chmod +x bin/dev from "."
   identical  app/assets/config/manifest.js
   identical  package.json
      remove  app/assets/stylesheets/application.css
   identical  app/assets/stylesheets/application.scss
   identical  app/javascript/application.js
   identical  app/views/layouts/application.html.erb
       exist  app/assets/builds
         run  yarn from "."
$ git status 
On branch main 
nothing to commit, working tree clean 

A follow-up PR will document this feature and add a bin/template script to newly bootstrapped projects to make it easy to re-run the template.

[tvararu]

I don't think this will fully work actually for certain files, like package.json, after we add more deps. Going to see what we can do about that.

Later edit: fixed!