v3 - add 'clean cache' script

[tiptronic]

add a dedicated 'npx app clean' script to get rid of the current cache

[scriptPilot]

@tiptronic - What is the use case behind? As soon as you edit any files, the cache will be updated.

[tiptronic]

The current build behavior most often leaves traces of the former cache.

You can easily reproduce, if you just copy some (even unrelated) src files - or even replace your current 'src' directory.

E.g. Safari and Chrome keep their own caches, so they keep traces of the last cache valid.

So it happens, after swapping files, etc... the cache fails to update all traces....

If there's a 'clean cache' script, which forces those browsers to invalidate caches, the problem is sovled.