WIP: Allow markdown attribute lists to be used in render hooks

[bep]

Fixes #8270

[bep]

/cc @faraixyz re. attributes and hooks. Two comments re this:
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1. I have only added it to the heading hooks context. I don't see how links/images can ever receive attributes ...?
2. I have added as a map, which I think make sense, so you can do {{ .Attributes.class }} etc.

[faraixyz]

This is neat, thanks for working on this.

1. I guess headings makes sense for now. Ignoring links, headings and images, do the markdown attributes work on inline elements, or is it just for blocks?
2. That makes sense.

[bep]

, do the markdown attributes work on inline elements,

Blocks AND headings (which excludes links and images).

[github-actions]

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