Extend justfile file detection

[mkatychev]

Description

Extend justfile filetype detection

As explained on the just repo quickstart:

The search for a justfile is case insensitive, so any case, like Justfile, JUSTFILE, or JuStFiLe, will work. just will also look for files with the name .justfile, in case you'd like to hide a justfile.

the ftdetect in the sactioned justfile vim plugin defines the full globmatch:
https://github.com/IndianBoy42/tree-sitter-just/blob/main/ftdetect/just.vim

Checklist:

• I am adding a new extension to a language.
  • The new extension is used in hundreds of repositories on github.com
    • Search results for each extension:
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.just
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • official justfile examples
    • Sample license(s): CC0 1.0 Universal
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

[lildude]

Your sample is in the wrong place.

[mkatychev]

I'm guessing I need to move it up a directory? https://github.com/github-linguist/linguist/blob/master/CONTRIBUTING.md I looked through the Contribution guidelines and I'm actually not sure what samples/<lang>/filenames/ is for.

EDIT: ran bundle exec rake && bundle exec rake test locally and passed.

[lildude]

I looked through the Contribution guidelines and I'm actually not sure what samples/<lang>/filenames/ is for.

This is for samples of explicitly named files added to the filenames section of a language.

[lildude]

LGTM. Thanks.

Note: this PR will not be merged until close to when the next release is made. See here for more details.