Auto merge of #148763 - yotamofek:use-yarn, r=Kobzol

Use `yarn` instead of `npm` for JS/TS dependencies

Building on `@lolbinarycat` 's excellent work ( rust-lang/rust#148672 ).

`npm` apparently has certain edge-cases where it doesn't actually respect the lockfile it's fed, which has led to at least one [CI breakage](https://rust-lang.zulipchat.com/#narrow/channel/242791-t-infra/topic/CI.20keeps.20failing.20because.20of.20npm.20error/near/554420543).

`yarn` seems to not suffer from those same issues, and also has a `--frozen` flag for its `install` command

Author: bors