Allow to add to Alchemy's importmap from Rails application

[tvdeyen]

What is this pull request for?

Rails applications are technically engines, but not really. For example the engine_name is the config/application.rb modularized class name. Which makes it not nice for this interface from an app developers perspective.

Let's make the interface more explicit by using a options hash with readable keys.

Example

Rails.application.config.before_initialize do
  Alchemy.admin_importmaps.add({
    importmap_path: Rails.application.root.join("config/alchemy/importmap.rb"),
    source_paths: [
      Rails.application.root.join("app/javascript/components/product_select.js")
    ],
    name: "product_select"
  })
end

Notable changes

Previously introduced Alchemy.engine_importmaps gets renamed into Alchemy.admin_importmaps

Checklist

• [x] I have followed Pull Request guidelines
• [x] I have added a detailed description into each commit message
• [x] I have added tests to cover this change

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.95%. Comparing base (a1e0391) to head (4e6f189).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2884 +/- ## ========================================== + Coverage 95.94% 95.95% +0.01% ========================================== Files 230 230 Lines 6260 6258 -2 ========================================== - Hits 6006 6005 -1 + Misses 254 253 -1 ```

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