Adds headless XR support to Isaac Lab and refactors the AppLauncher:
* Fixed headless XR rendering with Kit XR's OpenXR extension
* Automatically enables AR mode when running headless
* Centralized XR-specific settings from code to Kit app configuration
files
* Added new `isaaclab.python.xr.openxr.headless.kit` configuration file
* Introduced `--xr` flag in AppLauncher for explicit XR mode control
(also supports `XR=1` environment variable)
* Device resolution for XR should default to CPU unless overridden
* In a separate commit, the AppLauncher configuration resolution is
broken down to resolve the flake8 C901 complexity violation

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- New feature (non-breaking change which adds functionality)

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