Unify FGD script duplicates "i" parameter for both import and export

[vrad-exe]

The unify FGD script specifies i as an alias for both import and export, which appears to now cause an error on Python 3.11 or otherwise after some update.

Traceback (most recent call last):
  File "C:\Users\Luke\Git\hammeraddons\src\hammeraddons\unify_fgd.py", line 1323, in <module>
    main(sys.argv[1:])
  File "C:\Users\Luke\Git\hammeraddons\src\hammeraddons\unify_fgd.py", line 1239, in main
    parser_imp = subparsers.add_parser(
                 ^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Luke\AppData\Local\Programs\Python\Python311\Lib\argparse.py", line 1188, in add_parser
    raise ArgumentError(
argparse.ArgumentError: argument mode: conflicting subparser alias: i