PSA: emscripten now requires npm install as of 1.39.7

[Sam Clegg]

As previously discussed on the mailing list we are moving towards using npm to pull in dependencies.

As of emscripten version 1.39.7 the closure compiler dependency now comes from npm.   For users of emsdk this should be transparent since `npm install` is run post-install.   For developers working with emscripten directly `npm install` or `npm ci` is now required to be run manually after checkout.

If all goes smoothly more dependencies will be moving to npm modules over time.

cheers

sam