Use rollup to build the injection scripts for the browser

[mansona]

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Right now we are building scripts that are injected into the browser with ember-cli, which means that the module system for those scripts is AMD. Going forward (in Vite) Ember will not be relying on AMD for modules so we should upgrade the injection scripts to follow the same pattern.

This is not intended to be a breaking change, but it will allow us to unlock top-level await support for async loading of real modules