Improve XrSession script documentation and consistency

[willeastcott]

Summary

• Add comprehensive class-level JSDoc documentation with features list and usage example
• Add @type annotations to all class properties for better TypeScript support
• Add @import for Entity type
• Fix export pattern to match other XR scripts (class declaration + separate export)
• Fix sky layer save/restore to preserve original enabled state in AR mode

Details

The XrSession script was missing class-level documentation and had inconsistent patterns compared to the other XR scripts (XrControllers, XrNavigation, XrMenu). This PR brings it in line with the others and adds a bug fix for the sky layer restoration.

Documentation

• Added comprehensive class-level JSDoc block explaining what the script does
• Added features list and usage example
• Added @type annotations to all previously undocumented properties

Consistency

• Changed from \export class XrSession\ to \class XrSession extends Script { } export { XrSession };\ to match other XR scripts

Bug Fix

• \disableSky()\ now saves the original \layer.enabled\ state before disabling
• 
  estoreSky()\ now restores the saved state instead of always setting \ rue\
• This fixes a bug where the sky layer would be incorrectly enabled after exiting AR if it was already disabled before entering AR