feat: cubic-bezier curve for animation (#2059)

* feat: bezier curve for animation

* fixes

---------

Co-authored-by: Ivan Molodetskikh <yalterz@gmail.com>

docs/wiki/Configuration:-Animations.md

@@ -84,14 +84,24 @@ animations {
 }
 ```
 
-Currently, niri only supports four curves:
+Currently, niri only supports five curves.
+You can get a feel for them on pages like [easings.net](https://easings.net/).
 
 - `ease-out-quad` <sup>Since: 0.1.5</sup>
 - `ease-out-cubic`
 - `ease-out-expo`
 - `linear` <sup>Since: 0.1.6</sup>
-
-You can get a feel for them on pages like [easings.net](https://easings.net/).
+- `cubic-bezier` <sup>Since: next release</sup>
+    A custom [cubic Bézier curve](https://www.w3.org/TR/css-easing-1/#cubic-bezier-easing-functions). You need to set 4 numbers defining the control points of the curve, for example:
+    ```kdl
+    animations {
+        window-open {
+            // Same as CSS cubic-bezier(0.05, 0.7, 0.1, 1)
+            curve "cubic-bezier" 0.05 0.7 0.1 1
+        }
+    }
+    ```
+    You can tweak the cubic-bezier parameters on pages like [easings.co](https://easings.co?curve=0.05,0.7,0.1,1).
 
 #### Spring