Closure Migration & Typescript-ification

The Closure Library has [entered maintenance mode](https://github.com/google/closure-library/issues/1214).

We're working with the Closure team on migrating our dependency to a new Github repo, containing minimal dependencies required for gRPC-Web.

In the same process, we hope to modernize our codebase into TypeScript, and overhaul our toolchain to use TS tooling, so we no longer have to depend on the [Closure Compiler](https://github.com/google/closure-compiler).