Add style validator

[krassowski]

References

Fixes #14794, includes #14789

Code changes

Adds stylelint-csstree-validator for CSS validation, fixes invalid values in CSS styles.

User-facing changes

• Shadow behind popovers is restored as in Fix incorrect CSS value for statusbar popover shadow #14789
• Animation in shortcuts now works again (not a fan though, we can probably remove it depending on what others think).

Backwards-incompatible changes

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[fcollonval]

Thanks @krassowski

I agree with you that the animation for the shortcut editor is not great. Feel free to remove it or not before merging.

[fcollonval]

@meeseeksdev please backport to 4.0.x

[t03857785]

Looks like the CSS changes in #14766 and #14854 will not pass the stylelint-csstree-validator validation as it does not allow the use of min, max or calc in place of exact values, not sure there is an easy way to replace these do we just want to use exceptions for these

[fcollonval]

@t03857785 please use a ignore line rule and if possible link to the csstree issue: csstree/csstree#164