Use `getHeadingList` from `marked-gfm-heading-id` plugin

[patrickpatrickpatrick]

What

Delete extractHeadingPageHeadings(). Write new Metalsmith plugin that adds headings to each file object after each markdown file has been processed.

Addresses issue https://github.com/alphagov/govuk-design-system/issues/3171.

Why

Remove extra implementation to make use of already implemented feature of marked-gfm-heading-id. This method of implementation means we don't have parse the markdown multiple times and removes a loop of all the processed files.

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[patrickpatrickpatrick]

Cheers for picking this one up. The new plugin looks neat. 🙌🏻

Given it does a little more than simple markdown rendering now and is responsible for extracting headings info as well, I'd be keen to keep the tests we had for extract-page-headings to validate that it does that part of the work correctly.

The tests of the metalsmith-lunr-index will validate some of that, but having specific examples for each feature of the heading extraction (aliases, excluding from navigation, excluding from search) will make it easier to understand what exactly the heading extraction is about.

Would you mind adding them back (that'll probably require the plugin file to move in a folder in a similar way that the previous plugin was set up), please?

Makes sense. Was able to just add them all back with minimal changes :)