style: Replace "internal" properties with "enabled_in"

This allows enabling properties in ua sheets and chrome differently.

The setup is:

  * enabled_in needs to be one of the four values:
    ["", "ua", "chrome", "content"]

  * "chrome" implies "ua", and implies that they're explicitly enabled.

  * "" implies the property will never be parsed.

  * "content" implies the property is accessible unconditionally, modulo a pref.

Experimental still keeps trumping over those when the pref is enabled.

This PR replaces uses of internal="" by enabled_in="ua" or enabled_in="".

This may seem that it changes behavior, but since the properties where I added
enabled_in="" already unconditionally error from parse it's not.

Next step is annotating chrome-only properties.

components/style/properties/longhand/table.mako.rs

@@ -16,4 +16,4 @@ ${helpers.predefined_type("-x-span",
                           products="gecko",
                           spec="Internal-only (for `<col span>` pres attr)",
                           animation_value_type="none",
-                          internal=True)}
+                          enabled_in="")}