Add meta key to the possible multi selection keys on NME

[carolhmj]

Related Forum Issue: https://forum.babylonjs.com/t/nme-multi-select-ux-unusable-on-mac/30517/8
On Mac, the CTRL key always opens the browser context menu, making multi-select impossible. This PR adds the meta keys (windows/cmd) to the keys that can perform multi-select.

PR to test: https://playground.babylonjs.com/#99SQQW

[azure-pipelines]

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[azure-pipelines]

Snapshot stored with reference name:
refs/pull/12657/merge

Test environment:
https://babylonsnapshots.z22.web.core.windows.net/refs/pull/12657/merge/index.html

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

https://babylonsnapshots.z22.web.core.windows.net/refs/pull/12657/merge/index.html#WGZLGJ#4600

Links to test babylon tools with this snapshot:

https://playground.babylonjs.com/?snapshot=refs/pull/12657/merge
https://sandbox.babylonjs.com/?snapshot=refs/pull/12657/merge
https://gui.babylonjs.com/?snapshot=refs/pull/12657/merge
https://nme.babylonjs.com/?snapshot=refs/pull/12657/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/12657/merge#BCU1XR#0