Restore heading ids

[paulrobertlloyd]

In #89, I said (emphasis added):

In updating to a version of marked post v5, we can no longer depend on having built in support for heading ids. These can still be added separately via marked’s options in a consuming package.

Turns out, that’s not quite true. Because we are providing a renderer, this is incompatible with the marked-gfm-heading-id extension.

Enable marked-gfm-heading-id before this extension, and its ids will be removed.

Enable marked-gfm-heading-id after this extension, and our heading classes will be removed.

So let’s provide heading ids in our extension, which we can create using our own slugger. This may not be as robust as the one marked previously included (which used github-slugger under the hood), but should cater to 99.9% of use cases.

(We might have been able to use github-slugger, were it not for the fact that this package exports a Common JS module for compatibility with the GOV.UK Prototype Kit.)