style: Initial support for the color-scheme CSS property

Add initial support for the color-scheme CSS property, allowing pages to
choose between light and dark system colors per-element, and such.

Things that are left to do so that this can be enabled by default:

 * Dark system colors on Windows / Android / Standins.
 * Dark Canvas/CanvasText/Link visited colors (which right now are set
   via PreferenceSheet).
 * Dark form controls in nsNativeBasicTheme.
 * Processing the color-scheme meta tag to fill-in
   Document::mColorSchemeBits.

But this seems like enough progress to be landable on its own.

Differential Revision: https://phabricator.services.mozilla.com/D120843

components/style/properties/longhands/inherited_ui.mako.rs

@@ -95,6 +95,17 @@ ${helpers.predefined_type(
     has_effect_on_gecko_scrollbars=False,
 )}
 
+${helpers.predefined_type(
+    "color-scheme",
+    "ColorScheme",
+    "specified::color::ColorScheme::normal()",
+    engines="gecko",
+    spec="https://drafts.csswg.org/css-color-adjust/#color-scheme-prop",
+    gecko_pref="layout.css.color-scheme.enabled",
+    animation_value_type="discrete",
+    has_effect_on_gecko_scrollbars=False,
+)}
+
 ${helpers.predefined_type(
     "scrollbar-color",
     "ui::ScrollbarColor",

components/style/properties/properties.mako.rs

@@ -1376,6 +1376,9 @@ impl LonghandId {
             LonghandId::FontStyle |
             LonghandId::FontFamily |
 
+            // color-scheme affects how system colors resolve.
+            LonghandId::ColorScheme |
+
             // Needed to properly compute the writing mode, to resolve logical
             // properties, and similar stuff.
             LonghandId::WritingMode |