style: Clean up button active system colors

This patch does three things:

 * Unifies mac and gtk's buttonactivetext system colors (unships mac's from
   content, but it was never meant to be exposed).

 * Simplifies the forms.css rules, since <input type=color>'s color property
   value doesn't affect its rendering in any meaningful way.

 * Adds a buttonactiveface color, which we'll use to provide dark backgrounds
   for buttons in Windows dark mode (and is good practice, since generally
   every text system color should have a corresponding background).

So as-is it shouldn't change content-exposed behavior (except we stop exposing
the -moz-mac-buttonactivetext to content), but it's a worthy cleanup.

Depends on D127246

Differential Revision: https://phabricator.services.mozilla.com/D127259

components/style/values/specified/color.rs

@@ -376,10 +376,12 @@ pub enum SystemColor {
 
     /// Used for button text when pressed.
     #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
-    MozGtkButtonactivetext,
+    MozButtonactivetext,
+
+    /// Used for button background when pressed.
+    #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
+    MozButtonactiveface,
 
-    /// Used for button text when pressed.
-    MozMacButtonactivetext,
     /// Background color of chrome toolbars in active windows.
     MozMacChromeActive,
     /// Background color of chrome toolbars in inactive windows.