Add support for custom scripts

[Lucca-mito]

Summary

First-class support for adding custom scripts to a DocC-generated website. These may be local scripts, in which case the website will continue to work offline, or they may be external scripts at a user-specified URL. This support is in the form of a custom-scripts.json file, the scripting analog of theme-settings.json.

Full pitch: https://forums.swift.org/t/pitch-support-for-custom-scripts-in-docc/75357.

Dependencies

Corresponding change in swift-docc.

Testing

• Unit tests added.
• To test the feature end-to-end, you'll need to use the version of swift-docc with the corresponding changes. Follow the steps in the pitch for adding any custom scripts you want to your documentation website (your own scripts, or a library such as MathJax or D2). If you want a specific example to test the implementation:
  • Copy the custom-scripts.json and the custom mathjax-config.js script (shown in the pitch) to your documentation catalog.
  • Add LaTeX expressions to your in-source documentation.
  • docc convert the documentation catalog with your custom scripts. Observe that the modified swift-docc copied custom-scripts.json and the custom scripts into the documentation archive.
  • Preview the documentation archive. Observe that the LaTeX expressions are rendered on the page.
• Live demo: see here and [here](https://lucca-mito.github.io/swift-science/documentation/science/swift/collection/samplevariance()). The LaTeX expressions in the documentation text are dynamically rendered into equations via custom scripts.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• [X] Added tests
• [X] Ran npm test, and it succeeded
• [X] Updated documentation if necessary
  • Existing documentation does not need to change. New documentation for this feature will be added after the design and implementation are finalized.