style: Split TimingFunction into a separate file to match spec.

TimingFunction is defined in a separate spec (i.e. css-easing), instead
of transform, so we move it into a different file.

Depends on D9310

Differential Revision: https://phabricator.services.mozilla.com/D9311

components/style/values/computed/easing.rs

@@ -0,0 +1,11 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+//! Computed types for CSS Easing functions.
+
+use values::computed::Number;
+use values::generics::easing::TimingFunction as GenericTimingFunction;
+
+/// A computed timing function.
+pub type TimingFunction = GenericTimingFunction<u32, Number>;