ln -F is relevant only when the target is a directory, in which
case `ln` will delete the directory so that the link can occur.
In fisherman, we use links in only two places:
1. Linking files into .config/fish/{functions,completions}, and
2. Linking local plugins into the .config/fisherman
If my calculations are correct, we don't need -F in either case.
Both {functions/completions} never contain directories, so (1)
is probably safe. The fisherman config only contains directories
to installed plugins, so not using -F actually prevents the user
from overwriting a plugin currently installed with a local plugin.