[Editor] Add a color picker with predefined colors for highlighting text (bug 1866434)

The doorhanger for highlighting has a basic color picker composed of 5 predefined colors
to set the default color to use.
These colors can be changed thanks to a preference for now but it's something which could
be changed in the Firefox settings in the future.
Each highlight has in its own toolbar a color picker to just change its color.
The different color pickers are so similar (modulo few differences in their styles) that
this patch introduces a new class ColorPicker which provides a color picker component
which could be reused in future editors.
All in all, a large part of this patch is dedicated to color picker itself and its style
and the rest is almost a matter of wiring the component.

web/app_options.js

@@ -158,6 +158,11 @@ const defaultOptions = {
     value: 0,
     kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
   },
+  highlightEditorColors: {
+    /** @type {string} */
+    value: "yellow=#FFFF98,green=#53FFBC,blue=#80EBFF,pink=#FFCBE6,red=#FF4F5F",
+    kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
+  },
   historyUpdateUrl: {
     /** @type {boolean} */
     value: false,