Audits: Add a report on unused or undefined custom properties.

[ryelle]

⚠️ Don't merge ⚠️ This is just for use while working on ryelle/wordpress-develop#2

Try adding a report to identify custom properties that are defined but not used, and properties that are used but never defined.

Right now the output is:

Custom Properties audit

Defined but unused custom properties

--wp-admin--admin-bar--height
--wp-admin--theme--primary--step-10
--wp-admin--input--border
--wp-admin--input--border--focus
--wp-admin--input--background
--wp-admin--input--background--readonly
--wp-admin--checkbox--fill
--wp-admin--tabs-panel-active--box-shadow-from--focus
--wp-admin--tabs-panel-active--box-shadow-to--focus

Undefined custom properties

--wp-admin-theme-color
--wp-admin--admin-menu--icon--color--highlight
--wp-admin--tabs-panel-active--box-shadow--focus
--wp-admin--surface--border-2
--wp-admin--surface--background--alternate
--wp-admin--media-widget-preview--video--background