Implement Animation Blend Shape Tracks

* New track type BLEND_SHAPE
* Blend shapes are imported via this new track type
* Processing is more optimized (no longer relies on variants)
* Modified the Blend Shape API in MeshInstance3D to use indices rather than StringNames (more optimizes)
* Promo: Fixed a small bug in gizmo updating in Node3D that affected performance

Dedicated BlendShape tracks are required for both optimization and eventually implementing them in animation compression.

editor/icons/KeyTrackBlendShape.svg

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