Customizer: Allow arbitrary custom CSS

[sirreal]

• Allow arbitrary text in Customizer custom CSS.
• Protect the CSS data from mangling by KSES HTML filters.

Under some circumstances KSES would run post content filters and change the resulting content like this:

 @property --animate {
-  syntax: "<custom-ident>";
+  syntax: "";
   inherits: true;
   initial-value: false;
 }

This depends on #10656 to ensure styles output is safely printed in the HTML (merged here).

Trac ticket: https://core.trac.wordpress.org/ticket/64418

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[github-actions]

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Props jonsurrell, westonruter.

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[github-actions]

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[westonruter]

Just a couple trivial suggestions, but otherwise this looks great.