Add `.pacscript` to Shell

[Elsie19]

Description

This will add the .pacscript (similar to a PKGBUILD) file extension to Shell.

Checklist:

• [x] I am adding a new extension to a language.
  • [x] 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*.pacscript
  • [x] I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • alacritty.pacscript
    • Sample license(s):
      • MIT

[lildude]

Please update the PR template to use the section for adding a new extension. You can copy if from https://github.com/github-linguist/linguist/blob/main/.github/PULL_REQUEST_TEMPLATE.md

[Elsie19]

Please update the PR template to use the section for adding a new extension. You can copy if from main/.github/PULL_REQUEST_TEMPLATE.md

Should be good to go now.

[lildude]

The link to the source needs to be to the original source, not your fork of Linguist. You've also missed off the line stating the license of the sample.

[Elsie19]

Third times a charm 🤞🏻