Add WebXRInterface.xr_standard_mapping flag to attempt to convert button/axis ids to match other AR/VR interfaces

[dsnopek]

The button and axis ids used by WebXR are different than other AR/VR interfaces, due in part to the fact that WebXR can handle input from a wider range of devices and input methods.

However, as discussed with @BastiaanOlij on Discord, it is possible to convert from WebXR's ids to the standard Godot ids in some cases, which is what this PR does.

The conversion will happen if xr_standard_mapping property on WebXRInterface is set to true. It's set to false by default, for backward compatibility, however, I could be convinced to use the opposite default.

In cases where we can't convert the ids, they are just passed through directly from WebXR. And, when we are converting, if there's more data than we know how to deal with, it's simply discarded.

Note: This PR is only against 3.x, similar to other recent WebXR PRs. Once it's possible to use WebXR on Godot 4.x, this functionality will be ported there.

Related to #59949

[BastiaanOlij]

This makes it a lot more portable to create a game that can move between platforms without having to worry about differences in button assignments, so great change!

[dsnopek]

This should be ready now!

FYI, I removed the cherrypick:3.4 tag. It would probably be safe to port this to 3.4 (because backcompat is retained by default), but these changes touch some code that only exists in 3.x, and there really isn't a huge rush to get this out. I think it would be fine for this to come out in 3.5 with the other WebXR changes.

[akien-mga]

Thanks!