Animation: Shear operator for Graph Editor

This is a combination of two PRs from Ares Deveaux: #106521 and #106522

This adds a new operator that allows shearing keys
based on the position of the segment ends.
By pressing `D` while the operator is in modal you can
switch if the operator takes the left or the right segment end as a reference.

Co-authored-by: Ares Deveaux <aresdevo@gmail.com>
Pull Request: https://projects.blender.org/blender/blender/pulls/111735

scripts/startup/bl_ui/space_graph.py

@@ -302,6 +302,7 @@ class GRAPH_MT_key_blending(Menu):
         layout.operator("graph.blend_offset", text="Blend Offset")
         layout.operator("graph.blend_to_ease", text="Blend to Ease")
         layout.operator("graph.match_slope", text="Match Slope")
+        layout.operator("graph.shear", text="Shear Keys")
 
 
 class GRAPH_MT_key_smoothing(Menu):