Refactor of glTF extension handling

[GSterbrant]

Description

Refactors the way we deal with glTF extensions, giving us a better overview of the extensions we support and moving the inline extension handling to their own functions.

[mvaligursky]

Looks good to me.
Next step could go even further, similar to what I've done with other large files in the engine: create a folder called glb-parser, and move this file into it. Then create a file per extension in the same folder.

[GSterbrant]

Yeah, that could be a good plan to prepare for the potentially big library of extensions we will support.