Include user provided theme-settings.json file in compiled output

[mportiz08]

Bug/issue #, if applicable: #336

Summary

If a documentation catalog contains a theme-settings.json file, DocC will now include this file in the compiled output, alongside the JSON data and static renderer assets.

If a default theme-settings.json file is already provided by the renderer, the file provided by the catalog will take precedence.

This is the minimal DocC implementation work needed to support the proposal described in the Customizing the look and feel of Swift-DocC-Render forum post.

Dependencies

There are no explicit dependencies for this PR since DocC-Render already has some (minimal) support for the presence of this file in the output, however future PRs for DocC-Render will enhance these capabilities and document the supported settings.

Testing

Steps:

1. Download/unzip with-settings.diff.zip
2. Run cd swift-docc; git apply with-settings.patch
3. Run env DOCC_HTML_DIR=$(dirname $(xcrun --find docc))/../share/docc/render bin/preview-docs DocC¹
4. Open http://localhost:8000/documentation/docc and verify that some of the text on the page is now green

¹ this env var value will only work if you have a recent Xcode installed, however you can substitute it with a built copy of the renderer in other ways if needed

Checklist

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

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary (note: documentation for the JSON file itself will be added in a later PR here or in swift-docc-render)

[franklinsch]

Nice testing!

[mportiz08]

@swift-ci test