Clean up StartXR script

[Malcolmnixon]

This PR improves the start_xr script by:

• Exposing target XR frame rate
• Removing obsolete auto_initialize
• Using modern signal.emit() syntax
• Setting physics rate on WebXR
• Enabling WebXR hand-tracking if enabled in project settings
• Enabled WebXR passthrough support

Note: This is in preparation for improving the passthrough blend-mode support as a future PR.

[BastiaanOlij]

Looks good!