[Import] Add "skip file" import option to skip (and exclude from export) importable formats, auto set it for the images used by bitmap font.

[bruvzg]

As mentioned in #87971 and #82254, bitmap fonts use external images, but can't use these images as external resources since in many cases image preprocessing is required.

This PR:

• Adds "skip file" import option to skip (and exclude from export) importable formats (e.g, images).
• Automatically set "skip file" import mode for the images used by bitmap font, to prevent exporting these images twice (as source Image/Texture2D end embedded processed image in the fontdata).

[akien-mga]

Should this be:

Suggested change

	import_as->add_item(TTR("Skip File (No Import)"));
	import_as->add_item(TTR("Skip File (No Export)"));

?
Or "No Import nor Export" but it starts being quite lengthy.

[bruvzg]

Not sure if No Import part is necessary, it's not adding any information. Maybe Keep File (exported as is) and Skip File (not exported)?

[bruvzg]

Renamed options to: Keep File (exported as is) and Skip File (not exported).

Also added some doc notes: godotengine/godot-docs#9089

[akien-mga]

Makes sense to me, the added "Skip File" option seems potentially useful in other cases (e.g. source .blend files when you already have a .glb for the same scene).

I see we're lacking documentation about the existing "Keep File", would be good to add details about both options in https://docs.godotengine.org/en/latest/tutorials/assets_pipeline/import_process.html#changing-import-resource-type

[Zireael07]

(e.g. source .blend files when you already have a .glb for the same scene).

oh yeah please

[akien-mga]

Thanks!