Rename .import to godot-specific equivalent

[Repiteo]

The file extensions utilized by godot generally have a godot-specific naming scheme. That is, they wouldn't reasonably be in a scenario where the same extension name is explicitly used by another program or task. The primary examples are .gd, .godot, .gdshader, .gdshaderinc, and .gdextension. There's also a few file extensions that, while not explicitly linked, are still unique to godot. Examples of this include .tres and .tscn, signifying "text resource" and "text scene" respectively. Any filetypes that are more generic are relegated to the .godot folder, such as .cfg files

The one exception to this is .import, which has no explicit connections to godot nor a guarantee of not being used elsewhere thanks to its generic name. Practically speaking, this is harmless, but it does mean that other tools/utilities might have to jump through extra hoops to recognize that they're meant to be parsed the same way as godot resources. While this is a largely negligible concern, the point remains that .import is (to my knowledge) the only extension that this applies to. Making the extension instead .gdimport (or some equivalent) would make the naming schemes consistent