Migrate Closure to be shipped via emsdk or manually installed via npm.

[juj]

This removes Closure from the tree, and migrates it to be installed by running one of:

npm install -g google-closure-compiler (in any directory)
npm install google-closure-compiler (in Emscripten root directory)
emsdk install latest (all releases get closure by default)
emsdk install closure-compiler-20191111 (explicit install of the Tool)

Needs emscripten-core/emsdk#403 to land first.

[juj]

@sbc100 I think I'll need your help wrangling the CI, not sure how it relates to emsdk nowadays?

[sbc100]

I think we should add it to the package.json file a the top level.

I think we should have npm install run by emsdk as a post-install step. I'll take a look at that.

[juj]

Superceded by #9989.