The following properties accept <alpha-value> [1], which is
"<number> | <percentage>", so we update the parser, spec links, and
their web-platform-tests.
1. opacity
2. flood-opacity
3. fill-opacity
4. stroke-opacity
5. stop-opacity
6. -moz-window-opacity

Besides, shape-image-threshold [2] still only accepts <number>, so we need
to support a different version of `Opacity::parse()`.

[1] https://drafts.csswg.org/css-color/#typedef-alpha-value
[2] https://drafts.csswg.org/css-shapes/#shape-image-threshold-property

Differential Revision: https://phabricator.services.mozilla.com/D37493