feat: add diagonal backward strafe animations

[jb0gie]

Description

Adds dedicated diagonal backward strafe animations for walking and running, replacing the blended animations (right+back, back+left) with proper
diagonal movement clips.

Changes:

• Added WALK_BACK_LEFT and WALK_BACK_RIGHT emotes (2.2x speed)
• Added RUN_BACK_LEFT and RUN_BACK_RIGHT emotes (2.0x speed)
• Updated updateLocomotion() in createVRMFactory.js to use dedicated diagonal poses instead of blending two animations
• Added 4 new animation files: mp-walk-backleft.glb, mp-walk-backright.glb, mp-jog-backleft.glb, mp-jog-backright.glb

This improves visual fidelity when players move diagonally backward (e.g., pressing S+A or S+D), giving smoother, more natural character movement.

Type of change

• Component enhancement

How Has This Been Tested?

• Component functionality tests
• World integration testing

Test Configuration:

• Hyperfy Version: latest dev branch
• Test world setup: Local development world with player movement
• Browser(s): Chrome
• Node.js version: 20.x

Checklist:

• My code follows Hyperfy's component architecture
• I have performed a self-review
• I have documented the component's usage
• I have tested the component in multiple scenarios
• My changes maintain compatibility with existing worlds
• I have updated the component documentation if needed