Make the Pen tool use Ctrl to lock the angle of handles such that they maintain colinearity

[0SlowPoke0]

Partly closes #1870

• Ctrl locks the current angle of the handle (already implemented), but should also work to be colinear with the opposite handle if being dragged out of an existing anchor point, or if that anchor point has no handle then it should use the angle of the actual curve segment entering that handle, i.e. its tangent angle at its endpoint
• Added feature visualizing the 15° increment locking with Shift discord

Also fixes these bugs:

• Fixes scaling the handle to zero length rotates the opposite (Discord)
• Fixes dragging a handle into its anchor make the opposite rotate (Discord)

[Keavon]

!build

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[Keavon]

!build

[github-actions]

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