Closure Migration & Typescript-ification

[sampajano]

Closure library is pentering maintenance mode](google/closure-library#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.