Run a single, top-level tsc --build

[jeffposnick]

R: @tropicadri

TypeScript has support for project references, which work well for monorepos, like our own. We've been configuring our tsconfig.json files for use with project references, but we haven't actually been taking advantage of them from a compilation perspective. Instead, our build process would start a new tsc --build for each sub-package. This is partly because it was easier to migrate to TypeScript if we kept our old per-package build process in place, but now that we're almost all TypeScript, it can be improved.

This PR removes all of the per-package tsc --build invocations in favor of a single tsc --build run with our top-level tsconfig.json, which includes all of the project references.

This PR also stops cleaning up the TypeScript build artifacts from the previous build prior to starting a new build, as the composite setting means that they can be used to speed up subsequent builds. This is useful when running gulp build a few times in row while testing out some changes. I've moved the clean-up process to the gulp publish task flow, to ensure that nothing out of date ends up inadvertently published. (I haven't tested this flow yet, so this is something to keep an eye on the next time we cut a release.)

With these changes, I've seen gulp build drop from around 75 seconds to 30 seconds on my laptop.