TypeScript conversion so we have accurately generated types for consumers

These 3 PRs should be merged first:

As ember has first-class TypeScript support out of the box (almost), we need to to have all core addons also support that.

Like always, consumers don't need to use TypeScript, but our library code using TypeScript will provide better intellisense for both JavaScript and TypeScript users.

Incidentally, achieving more alignment with the TS-v2-addon blueprint, we gain support for Glint-using projects.

This work continues the work of https://github.com/ember-cli/ember-page-title/pull/246

Demo in the docs app:

Changes:

convert to TS
npx ember-apply typescript to the docs app
set up Glint in the docs app
test with Glint in the docs app
upgrade the test-app to latest ember-source -- (fastboot + embroider) is not bug free with ember-source 3.28, but is with latest ember-source. Going to inquire with the embroider folks if this combo should be supported. It could also be that the 3.28 folks need to use an earlier version of embroider -- idk how complex / thorough we want the test matrix to be

ember-cli / ember-page-title

TypeScript conversion so we have accurately generated types for consumers #275