improve dbux-projects webpack'ing

[Domiii]

Webpack Config

• unify API for @babel/register, webpack, Jest etc. include/ignore and cache settings #621
• add support for babel-loader -> cacheDirectory
  • consider enabling it by default (e.g., we enabled it explicitely in react-tic-tac-toe)
• add pw and pb via rule test, include and exclude options
• test all of the above with react-tic-tac-toe
  • -> encoding of click event data takes 21s
• monitor for Webpack build errors
  • add error hook to webpack.config.dbux.base.js - explanation (NOTE: V4 only)
    • use webpack's Node API: compiler.watch(options, callback)?
• while executing startWatchModeIfNotRunning, show a webpack-specific message in progress bar
  • (so as to let users know what is taking so long)
• Q: when switching from one exercise to another, and they are of the same project: can we not stop and then re-start webpack?
  • → Nah, too risky.

UI

• add a button to reflect webpack status for webpack-enabled projects
  • button can also be used to stop (or otherwise control) webpack

[Domiii]

Related: #627