Introduce `allowJs` and `checkJs` compiler options

[peterblazejewicz]

The common scenario is to migrate existing code to TS and start with TSC to compile/check existing code before switching completely to TS.
Have you considered adding that option, so it could be used when migrating code to Angular?

Thanks!

[vsternbach]

It's possible I guess, but it would take quite an effort to update all of the webpack configuration to support js and ts simultaneously, unfortunately I don't have time for that, any PR would be welcome of course)