Replace 12 year old CSS colour parsing library

The replacement library from https://colorjs.io/ is much, much larger but I think we can develop a custom build that uses treeshaking to whittle the code down to the bits that we need. @linonetwo does that sound feasible?

I intend the explore further improvements but I wanted to start by establishing a library that can do modern P3 and OKLCH colour calculations.

Author: Jermolene

core/modules/utils/dom/color-utils.js

@@ -0,0 +1,31 @@
+/*\
+title: $:/core/modules/utils/color-utils.js
+type: application/javascript
+module-type: utils
+
+Color.js related utilities
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+exports.parseCSSColor = function(colourString) {
+	var Color = require("$:/core/modules/utils/dom/color.js").Color,
+		c = null;
+	try {
+		c = new Color(colourString);
+	} catch(e) {
+		// Do nothing on an error
+	}
+	if(c) {
+		var rgb = c.srgb;
+		return [rgb[0],rgb[1],rgb[2],c.alpha];
+	} else {
+		return null;
+	}
+};
+
+})();