Add FileAccess.store_{buffer,string}_as_file()

[timothyqiu]

Closes godotengine/godot-proposals#13110

These two static methods are the opposite of FileAccess.get_file_as_{buffer,string}(). The "store" vs "get" naming is consistent with the non-static version.

The parameters follow the same order as ResourceSaver.save() (as the order was discussed and swapped in 4.0), and are also in the same order as mentioned in the method name.

[bruvzg]

Code looks fine. Not sure if it's worth adding methods for two lines of script, but since we have get_ variants, it probably makes sense to add store_ as well.