chore: leverage babel watcher

Up till now, we've only used a typescript watcher and a webpack watcher. This PR adds in the babel watcher.

TODO: not covered by this PR is auto-regenerating the prescan.json (npx kui-prescan) in a watchy fashion. To do so would I think require somehow hooking into the babel watchers. Because we can only regenerate prescan after babel is done. I don't see (yet) a way to hook into babel in this fashion, i.e. a post-compile watch hook. I did look into using webpack for this, since it does have a notion of post-watch and post-compile; but there is still a race between babel, which results in massive instability. I also contemplated hooking into the typescript watcher (via tsc-watch); we could use this, but it would require rebuilding all of babel on every tiny typescript change. So, for now, one must still rerun npx kui-prescan manually, every time a new kui command registration is added.

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kubernetes-sigs / kui