Improved CI checks of JavaScript code

[ravicious]

A collection of tasks, each of them in theory can be completed within a few hours. Hence why #21075 is not included here, because enabling strictNullChecks is a big undertaking.

Mid effort as I imagine it to be a cumulative score for all tasks. Mid value since some of us definitely spent some time chasing bugs and errors which would've been caught by those checks, but it's not like this will make us fundamentally faster.

Tasks

Beta Give feedback

1. Enable no-floating-promises ESLint rule #41945
   developer-experience effort:mid value:low +3
2. Enable plugin:react/recommended ESLint rules #34329
   developer-experience effort:low ui value:low +4
3. Enable noImplicitThis setting of TypeScript #34192
   developer-experience effort:low ui value:low +4