(Worklet) Add cornerbox demo

[jerosoler]

Demo: https://jerosoler.github.io/cornerbox/

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[jerosoler]

@googlebot I signed it!

[una]

Thanks for this PR! I love this demo! A few questions:

1. I notice that the corners are still slightly visible, even when --cornerbox-line is at 0. Is it possible to take this full off-canvas?
2. I would have the range be from 0 for --cornerbox-line because I think this is a neat effect that can animate in
3. The second variable --cornerbox-lenght has a typo, but might also make more sense to be called the --cornerbox-width instead. Similarly, I might rename --cornerbox-line to be --cornerbox-length, because it feels more descriptive

[jerosoler]

Hello @una

Thanks a lot! I like that you like it!

I have uploaded the corrections you indicate. You're right ;) Good work!

[una]

Nice one!