fix!: align Fish entrypoint behaviour with other shells

[hyperupcall]

Summary

As per this comment, this matches the behavior of Fish with Bash, Nushell, and all the other shells. Along with a few other things

There were some bugs with the previous Fish implementation that are now fixed:

• If ASDF_DIR was set to an empty string, the value was used (unlike in Bash/Zsh, Elvish, Nushell, which replaces it with a correct one)
• ASDF_DIR is now always an absolute path to the asdf directory (previously it could have been a .). (Since it's an absolute path, we now export it so that asdf can use it instead of recalculating)
• Depending on version, either PATH or fish_user_paths (through fish_add_path) would be modified. Now, only the latter is potentially modified
• Flag forms to the status builtin like -f are now deprecated in Fish and have been replaced with the alternative (string filename)

It seems that fish_add_path potentially modifies the placement of an entry in $fish_user_paths, even if --move is not specified, so I opted to prepend to $fish_user_paths manually, so the behavior is consistent with all the other shells.

Now we also use fish_indent to make sure Fish scripts are formatted nicely (even if we have relatively few of them)

[jthegedus]

🎉

[jthegedus]

Since this is run locally, it would be ideal to check Fish is available or error with a warning to the user that it is required.

Even more ideal would be to get fish_indent as a standalone CLI tool and manage it with our .tool-versions file so people get the same experience as with shellcheck and shfmt in this repo.

A separate Issue tracking this would suffice I would say.

[jthegedus]

I don't use fish anymore so haven't tested this locally myself, but LGTM

[jthegedus]

Merging to get people testing this.