Embroider compatibility

[bobisjan]

This is a tracking issue for future Embroider compatibility.

Based on the Embroider's SPEC.md, how this add-on should be refactored away from using treeForPublic() hook in the future, while support to the following should remain

• read configuration located at config/manifest.js,
• produce manifest.webmanifest (and browserconfig.xml ) at build-time based on the configuration,
• fingerprint URLs in that files,
• fingerprint that files?

The SPEC refers to use Build-time Conditionals, but they seem related to JavaScript and Templates only.

/cc @ef4