style: Move legacy -moz-box collapse to its own CSS property

Make display: -moz-box's visibility: collapse handling switchable by its
own CSS property.

Longer term maybe we should switch the front-end away from visibility:
collapse altogether (there are some alternatives), but for now this will
allow to move the front-end to switch to modern `display: flex` while
keeping `visibility: collapse` work as in -moz-box.

Differential Revision: https://phabricator.services.mozilla.com/D171472

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

@@ -45,6 +45,16 @@ ${helpers.single_keyword(
     servo_restyle_damage="rebuild_and_reflow",
 )}
 
+${helpers.single_keyword(
+    "-moz-box-collapse",
+    "flex legacy",
+    engines="gecko",
+    gecko_enum_prefix="StyleMozBoxCollapse",
+    animation_value_type="none",
+    enabled_in="chrome",
+    spec="None (internal)",
+)}
+
 ${helpers.single_keyword(
     "-moz-box-layout",
     "flex legacy",