Ensure embroider-optimized compatibility

[elpoelma]

Overview

This PR includes some changes to ensure embroider-optimized compatibility:

• Some changes to the webpack configuration and exporting a webpack config object for host apps in webpack-config.js'
• Update ember-resolver to 11.0.1
• Addition of @ember/string as a dev- and peer-dependency. (see https://deprecations.emberjs.com/v4.x/#toc_ember-string-add-package)
• Include explicit .js extension for process/brower.js webpack plugin to ensure esmodule compatibility
• Explicitly import ember-velcro component to workaround embroider issue. (see lblod/ember-rdfa-editor@c8e8de8)

Additionally this PR includes the following changes:

• Addition of ember-power-select as a peerdependency. It specifies the 6.x and 7.x ranges.

How to test/reproduce

• Run the embroider-optimized scenario (ember try:one embroider-optimized) and ensure no errors occur
• Ensure that the dummy application still works as expected.

Checks PR readiness

• UI: works on smaller screen sizes
• UI: feedback for any loading/error states
• Check if dummy app is correctly updated
• Check cancel/go-back flows
• changelog
• npm lint

[elpoelma]

Update: the embroider-safe scenario now succeeds without errors.

[elpoelma]

Once lblod/ember-rdfa-editor#995 is merged, embroider-optimized will also succeed. I changed the base to that PR as it is need for embroider-optimized

[elpoelma]

@piemonkey @abeforgit I don't think this PR is really breaking?