Add support for CMD key to KeyboardEventModifier

[KevinKibler-Lovelace]

Feature

On MacOS, the CMD key is roughly (but not exactly) analogous to the CTRL key on Windows. Many keyboard shortcuts that use CTRL + on Windows use CMD + on MacOS.

Add META to the KeyboardEventModifier object to better support Cesium on MacOS.

[KevinKibler-Lovelace]

In this file: https://github.com/CesiumGS/cesium/blob/main/packages/engine/Source/Core/KeyboardEventModifier.js

[jjspace]

Just as a note, we rely on browser support for keyboard modifiers. To support Command we will need to add KeyboardEventModifier.META not CMD because browsers consider Command as the Meta key KeyboardEvent.metaKey