Update the Generator for v15

[justin808]

• Switch to use npm not yarn
• Install shakapacker if not installed
• Update the generator to use auto registered bundles
  * https://github.com/shakacode/react_on_rails/blob/f17b4a02d917acb9e51dbacb2437bf8e08d2ebca/docs/guides/file-system-based-automated-bundle-generation.md
  * https://github.com/shakacode/test-react-on-rails-v15-hello-world-on-rails-v8 for an example app

[justin808]

Let's use this bin/dev file: https://github.com/shakacode/test-react-on-rails-v15-hello-world-on-rails-v8/blob/main/bin/dev for both spec/dummy and the generator.

[ihabadham]

both spec/dummy and the generator.

In #1769 you only used it for the generator and not in spec/dummy: https://github.com/shakacode/react_on_rails/pull/1769/files#diff-f08d14e7c9fd267c91103c9416a309782a924ab3d176b9f334a5a001b2c5e831R9

was that intentional? i used it for spec/dummy in #1770 here: https://github.com/shakacode/react_on_rails/pull/1770/files#diff-f08d14e7c9fd267c91103c9416a309782a924ab3d176b9f334a5a001b2c5e831

[ihabadham]

also, any reason for removing the specs and only leaving a basic syntax guard in dev_spec.rb?

i kept them in #1770