fmt: insert auto imports after shebang

[ttytm]

input:

#!/usr/bin/env -S v
os.join_path('a','b')

formatting it:

current:

import os

#!/usr/bin/env -S v

os.join_path('a', 'b')

after:

#!/usr/bin/env -S v

import os

os.join_path('a', 'b')

[spytheman]

Excellent work.