Allow directory with -f flag

[itowlson]

Spin built-in commands that use the -f flag to point to an application usually accept directories as well as files - if the -f value is a directory, then it looks for a spin.toml in that directory. (E.g. spin up -f ~/testing/myapp/)

Plugins are, of course, not bound by Spin conventions; but this one is quite nice for the (hopefully) limited effort involved, so might be worth implementing for familiarity.