Add Just syntax highlighting

[an0nfunc]

Adds syntax highlighting support for Just command runner files (justfile, Justfile, .justfile).

Fixes #3256

The syntax file is adapted from nk9/just_sublime (MIT license) with modifications to use bat's built-in Python and Bash scopes for shebang recipe bodies, since syntect doesn't support the extends: feature.

[alloc33]

It's time to merge? Came here for just syntax and find out this PR on the next day after it's creation

[keith-hall]

Thanks for your contribution. I hesitate to merge this for two reasons:

• having the sublime-syntax directly in bat and not in a submodule puts the onus on us to maintain it
• I'm not convinced the MIT license has been respected - we need to keep the copyright notice and add attribution i.e. link to the original repository
• it would be nice to improve syntect with extends support so we don't need any workarounds

[an0nfunc]

Thanks for your contribution. I hesitate to merge this for two reasons:

* having the sublime-syntax directly in `bat` and not in a submodule puts the onus on us to maintain it

You prefer a submodule then? Besides the syntax thing it should be possible ofc.

* I'm not convinced the MIT license has been respected - we need to keep the copyright notice and add attribution i.e. link to the original repository

Will check that.

* it would be nice to improve syntect with extends support so we don't need any workarounds

Sure would, but thats kind of out of the scope of this PR. I can have a look at what that entails, but that's certainly not what I set out do to.