Feature: Inspector v2 hotkey support

[marns]

Goal: more extensible Inspector API for use in e.g. custom editors, specifically hotkeys for gizmos and access to selected entity.

Problem:
The inspector manages its own gizmo and selection states; thus setting these up externally would result in duplicate or unsynced states. These are currently not accessible by public API.

Proposed solution:
Expose a new IInspectorHandle with gizmo and selected entity controls, and a default hotkey implementation for gizmos

Examples:

• Updated Playground to use default hotkeys (QWERTX)
• Updated Sandbox to use custom hotkeys (`,1,2,3,4,0) in its existing hotkey event callback

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
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[bjsplat]

Snapshot stored with reference name:
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https://playground.babylonjs.com/?snapshot=refs/pull/17569/merge
https://sandbox.babylonjs.com/?snapshot=refs/pull/17569/merge
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If you made changes to the sandbox or playground in this PR, additional comments will be generated soon containing links to the dev versions of those tools.

[bjsplat]

You have made possible changes to the playground.
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Note that neither Babylon scenes nor textures are uploaded to the snapshot directory, so some playgrounds won't work correctly.

[bjsplat]

You have changed file(s) that made possible changes to the sandbox.
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[bjsplat]

Interaction tests

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Visualization tests for WebGPU

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[ryantrem]

Thanks @marns for working on this! Couple of questions:

1. Why can't keyboard handling be done directly in the GizmoToolbar? Why do you want to handle it outside?
2. Why do you want different shortcut keys for the GizmoToolbar in Sandbox vs. Playground?