Expose _rootUrl of gltf loader as public get property

[tholub99]

See https://forum.babylonjs.com/t/expose-gltfloader-rooturl-property-for-extensions/46958

[bjsplat]

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[bjsplat]

Snapshot stored with reference name:
refs/pull/14673/merge

Test environment:
https://babylonsnapshots.z22.web.core.windows.net/refs/pull/14673/merge/index.html

To test a playground add it to the URL, for example:

https://babylonsnapshots.z22.web.core.windows.net/refs/pull/14673/merge/index.html#WGZLGJ#4600

Links to test babylon tools with this snapshot:

https://playground.babylonjs.com/?snapshot=refs/pull/14673/merge
https://sandbox.babylonjs.com/?snapshot=refs/pull/14673/merge
https://gui.babylonjs.com/?snapshot=refs/pull/14673/merge
https://nme.babylonjs.com/?snapshot=refs/pull/14673/merge

To test the snapshot in the playground with a playground ID add it after the snapshot query string:

https://playground.babylonjs.com/?snapshot=refs/pull/14673/merge#BCU1XR#0