style: Part 1: Add animation-timeline longhand property in style system

This patch adds the animation-timeline longhand property. For
shorthand, we will do that in the next patch.

This patch includes the aut-generated code in
devtools/shared/css/generated/properties-db.js, by `./mach devtools-css-db`.

Note:
1. we will use this property in Bug 1676791. For now, only make sure
   we parse it and serialize it correctly.
2. The syntax of animation-timeline may be updated, based on the spec
   issue: https://github.com/w3c/csswg-drafts/issues/6674.
   However, it's not a big problem to update it later, so we still can
   prototype this property based on the current version of spec.

Differential Revision: https://phabricator.services.mozilla.com/D126450

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

@@ -320,6 +320,20 @@ ${helpers.predefined_type(
     rule_types_allowed=DEFAULT_RULES_EXCEPT_KEYFRAME,
 )}
 
+${helpers.predefined_type(
+    "animation-timeline",
+    "AnimationTimeline",
+    "computed::AnimationTimeline::auto()",
+    engines="gecko",
+    initial_specified_value="specified::AnimationTimeline::auto()",
+    vector=True,
+    need_index=True,
+    animation_value_type="none",
+    gecko_pref="layout.css.scroll-linked-animations.enabled",
+    spec="https://drafts.csswg.org/css-animations-2/#propdef-animation-timeline",
+    rule_types_allowed=DEFAULT_RULES_EXCEPT_KEYFRAME,
+)}
+
 <% transform_extra_prefixes = "moz:layout.css.prefixes.transforms webkit" %>
 
 ${helpers.predefined_type(