Closure Library deprecation

[emaxx-google]

Our codebase here relies a lot on Closure Library, which got deprecated and is about to be sunset: https://github.com/google/closure-library/issues/1214

[emaxx-google]

In the short term, we don't need to do anything as we pin the Closure Library to a version that we import and store under //third_party/closure-library/. So as long as there's no serious issue discovered (which we cannot fix in our copy) or incompatibility with future browser versions, we can in theory sit indefinitely on top of it.

For the long-term perspective, if we were to start migrating off it, the whole list of functionality used by us is pretty long, but what we rely most heavily upon is:

• logging,
• message channels,
• Disposable,
• the unit test and mock framework.

It seems that there's no ready-to-use list of replacement libraries for specific features, so the work would require analyzing these dependencies and finding analogs or reimplementing. As it was pointed out to me, this work might also require switching to more modern technologies like TypeScript, NPM, esbuild, etc.