Support Importmap and revise Webpack support

[mshibuya]

This PR adds ability to use Importmap (via importmap-rails) or Webpack (via jsbundling-rails), along with supporting CSS build (via cssbundling-rails) in Rails 7.0 applications.
Existing preliminary support for Webpack was revised in attempt to follow the jsbundling-rails/cssbundling-rails way.
This implementation should also be compatible with propshaft setup, instead of sprockets.

But please note that the Importmap implementation can't be used as it is, since it requires the npm package to be available for CDN delivery. I'll release the npm package soon, possibly as '3.1.0-alpha' or so.
Webpack implementation should work even now, so please try it and give feedback.

[yubele]

@mshibuya
Running rails rails_admin:install will fail. You need to check if the class exists and return an importmap.

rails_admin/lib/rails_admin/config.rb

Line 262 in d073c4d

def asset_source

I made the following modifications. I hope this will be helpful.

#3495

[mshibuya]

I don't understand the problem, what did you do and what did you get?
Did you try to do config.asset_source = :importmap without installing importmap-rails?

[yubele]

Did you try to do config.asset_source = :importmap without installing importmap-rails?

importmap-rails has been pre-installed.

config.asset_source = :importmap

I believe the above refers to RailsAdmin.config.asset_source.

I believe this problem occurs because $ bin/rails rails_admin:install has not yet created config/initializers/rails_admin.rb at the stage of running $ bin/rails rails_admin:install.

Here are the steps I followed

1. $ bundle add importmap-rails.
   2.$ . /bin/rails importmap:install.
   3.$ bundle add rails_admin --branch importmap --github railsadminteam/rails_admin
2. $ bin/rails rails_admin:install.

I got the following error in 4.

[Warning] After upgrading RailsAdmin to 3.x you haven't set asset_source yet, using :sprockets as the default.
To suppress this message, run 'rails rails_admin:install' to setup the asset delivery method suitable to you.
/usr/local/bundle/bundler/gems/rails_admin-d073c4dfd315/lib/rails_admin/engine.rb:53:in `block in <class:Engine>': uninitialized constant RailsAdmin::Engine::Sprockets (NameError)

        Sprockets.register_preprocessor 'application/javascript', RailsAdmin::ESModulePreprocessor
        ^^^^^^^^^
Did you mean?  Socket
               Process
               IPSocket

[mshibuya]

Ah I understood now, thanks for letting me know. Fixed by 705fd32 👍

[coveralls]

Coverage decreased (-0.05%) to 95.994% when pulling 425f2e3 on importmap into ce1bb5a on master.

[mshibuya]

Let me merge this in now, to prepare for the beta release.