Backport box-shadow support for blocks via theme.json

[andrewserong]

This PR backports the following Gutenberg PR: WordPress/gutenberg#41972. Note that box-shadow is already listed in PROPERTIES_METADATA so that line has already been backported.

See tracking issue: WordPress/gutenberg#43440

Trac ticket: https://core.trac.wordpress.org/ticket/56467

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[andrewserong]

CC: @madhusudhand and @scruffian, I was looking through the Theme JSON class, and it looked like this one hadn't been backported yet, and I assume it's intended to make it into 6.1?

[scruffian]

Should we also add it to remove_insecure_properties?

[andrewserong]

Thanks for taking a look @scruffian! I double checked, and it looks like we don't need to manually specify it in remove_insecure_properties because when remove_insecure_styles is called, it'll do the lookup in compute_style_properties to static::PROPERTIES_METADATA which already includes the box-shadow key. So, I believe it's already handled 👍

[cbravobernal]

LGTM - Tested and works as expected!

[desrosj]

Is there any way that we can write some unit tests for this? I feel like we have not kept up with the new settings and styles allowed in theme.json. Our test theme is missing quite a few of the items listed in the reference.

I think this fine to merge in as is, but I'd like to open a ticket to expand our theme.json testing.

[desrosj]

Merged in https://core.trac.wordpress.org/changeset/54253.

[desrosj]

Created Core-56611 for expanding unit tests.