Enhance Fish Shell Completions for Optional --justfile Argument

[rubot]

This pull request introduces a minor enhancement to the Fish shell completion script.

Changes:

• Added logic to dynamically set the JUST_JUSTFILE environment variable when the -f or --justfile argument is provided in the command line.

Motivation for the Change:

Currently, users specifying a custom justfile with -f or --justfile do not have this reflected in their completions within the Fish shell. This can lead to inconsistencies and an unintuitive user experience. By setting the JUST_JUSTFILE environment variable, the change is as minimal as it could be and completions will be more accurate by reflecting the correct justfile being used.

Further Notes:

• This change adds only three lines of code to enhance the existing Fish shell completions.
• It retains backward compatibility and only enhances the functionality for those who use a custom justfile.

Testing:

• Tested manually with various just commands in Fish shell, both with and without the -f/--justfile options, to ensure appropriate behavior.

I hope these enhancements provide a more seamless experience for just users leveraging Fish shell completions.

Thank you for considering this improvement!

[casey]

Nice, thank you for the PR! These kinds of things are very nice quality-of-life improvements for completion users.