SkyMesh: Add isSkyMesh flag and deprecate isSky

[querielo]

Added new isSkyMesh property for type testing and marked isSky as deprecated.
Looking at the SkyMesh class, here's why we added the isSkyMesh flag and deprecated isSky:

The class is called SkyMesh, so isSkyMesh provides a more precise type check than the generic isSky. This follows the naming convention where the flag matches the class name (Three.js commonly uses is[ClassName] flags for type checking).

// Before (deprecated)
if (object.isSky) { /* ... */ }

// After (recommended)
if (object.isSkyMesh) { /* ... */ }

[Copilot]

Pull Request Overview

This PR enhances type checking for the SkyMesh class by introducing a more precise isSkyMesh flag that follows Three.js naming conventions, while deprecating the less specific isSky property for backward compatibility.

• Adds isSkyMesh flag for precise type testing that matches the class name
• Deprecates isSky flag with clear JSDoc annotation
• Maintains backward compatibility during the transition period

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