chore: Refactor to use TypeScript & lerna builds.

[abritinthebay]

This is a first pass, with obvious improvements that can be made, but it's better than not being in TS.

• Set up project to extend monorepo's tsconfig.json
• Updated monorepo config to fix issues seen during conversion
• Added build and compile scripts
• Added ember-cli-typescript to allow (non app) code to be Typescript seamlessly
• Added TS-Node to allow TypeScript in code included in index.js for local development
• Added global (to addon) .d.ts file for the types needed (inc mocking up some of Ember's types)
• Pulled apart code into separate files for easier maintenance and reading
• Added a lot of guard logic
• Removed lodash.merge & replaced with ESNext/TS idioms

[abritinthebay]

Looks like playground is running into issues on CI with the new linked version of the cli addon. Will noodle on how to fix that.

[abritinthebay]

Fixed: needed to explicitly pass ts-node the tsconfig.json file, else it gets lost in the link/build process.

[abritinthebay]

Was feeling curious so cleaned this up a bunch based on what we discussed. There are still some custom types but they're explicitly imported (from _utils) and they may be able to be trimmed some more.