Choose normal map format at export instead of in preferences

[Silas Merlin 99]

I am always weary of changing the normal map export format in Preferences, because there is the (very strong) chance that I forget I did it.

In my opinion it would make more sense to be able to choose the normal map format at export (y+ or y-).
(Also, an option ticked by default to add an indication of the chosen format in the exported file name)

Thank you.