Colors Audit: Check that only colors in the palette are used

[ryelle]

This is an idea to improve the colors audit. Core has a fixed color palette, so we could use that list to highlight colors that are not in the set (if any). The palette should work like an allowed-list, flagging anything not on the list.

For example: if #f00 is added, it will show up in the audit, but currently no indication that it shouldn't be there. There could be another section of the Colors report, which lists these non-allowed-list colors:

1. #f00 found in wordpress/src/wp-admin/css/admin-menu.css, line #33.

[danfarrow]

That would be very useful!