[v2] Use Babel 7 transforms rather than tsc for transpilation

[dfreeman]

This PR constitutes the first chunk of work toward #333. It replaces our use of tsc with @babel/plugin-transform-typescript for transpiling TypeScript files in the host.

Some notes:

• I've added some basic local type declarations for ember-cli and converted the main addon module to TypeScript
• At this point, ember-cli-typescript only transpiles code and does no typechecking
• Most of the previous incremental-typescript-compiler code is still around for now; it will be cleaned up in the future once we have a good handle on typechecking
• The ts:precompile command now only emits .d.ts files, since the .ts ones are consumed normally by Babel
• Some test coverage has been removed
  • We're no longer testing MU code emitting: we would need to tie ourselves to ember-cli#master in order to actually test it, but since we're now just using a Babel plugin, the risk of breakage is much lower
  • We're no longer testing file shadowing behavior, as the expectations have changed (see the Breaking Changes section of ember-cli-typescript 2.0 #333 for details)
  • The "build fails on type error" acceptance test is temporarily disabled until we have typechecking again

[chriskrycho]

Confirmed this works in our ~60kloc TS app. No issues, some workarounds needed as expected.

[dfreeman]

I should add that I've gotten build + tests passing on a couple of open source projects as well

• ember-milestones (diff - no code changes!)
• ember-osf-web (diff - changes grouped by commit)

[dfreeman]

Since this has gotten an initial pass of testing, I'm going to merge this branch into v2 so that we don't wind up with an ever-lengthening chain of dependent PRs. Review is still welcome over on #333.