Intent to unship: -moz-prefixed multi-column properties and -moz-column-gap

[Ting-Yu Lin 林庭宇]

In bug 1308636 [1], I plan to unship all -moz-prefixed multi-column
properties defined in CSS Multi-column spec [2] and -moz-column-gap [3].

We've shipped column-span in Firefox 71 [4] and completed the support of
all the properties in the multi-column spec. Other standard properties have
been shipped since Firefox 52 [5]. We now have a solid foundation to unship
the prefixed ones.

I added a new pref "layout.css.prefixes.columns" and set its default value
to "false" to guard the prefixed properties behind. If any serious
web-compat issues arise, we can have those prefixed properties back.

Ting-Yu Lin (:TYLin)

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1308636
[2] https://drafts.csswg.org/css-multicol-1/
[3] https://drafts.csswg.org/css-align-3/#propdef-column-gap
[4]
https://groups.google.com/d/msg/mozilla.dev.platform/p80ptKoqVvk/uKJwoK6JDAAJ
[5]
https://groups.google.com/d/msg/mozilla.dev.platform/EammrHjrCpw/yXcERONyAgAJ