ESLint 9 and Promise checks merged into WebHare master

[Arnold Hendriks]

As suggested by Wouter in https://groups.google.com/a/webhare.dev/g/developers/c/6PMa-smkalU (just to make it clear where all blame should be placed) - we've now enabled various promise lintings. Also a good opportunity to upgrade to eslint 9

Most common new issues are still warnings - at some point we may turn them into errors, or allow modules to opt-in into making them errors. We may need to modify some APIs to make dealing with these new warnings easier.