Support anchor links in TOC block

[andybroomfield]

The TOC block works with ID's added to the H2, either automatically via auto heading ids Custom heading filter plugin.
It's also possible to add them in manually in the source editor.

When using CKEditor Anchor Link these are added either as invisible anchors or within the h2. Currently these won't be picked up by the TOC block.

This might be a common way authors add anchor links. We could either

• Support this first party in the TOC block
• Add an alter hook or event subscriber interface to allow custom modules to provide their own discovered anchor links.

[rupertj]

I think we could support this directly in the HeadingFinder and HeadingIdFilter. It could just be a fallback case in both - if we find a heading but it's got no id, also search inside that tag for an with an id and use that if found.

I don't think this'd break anyone's existing setup either. There's unit tests for the affected code though, so it should be easy to make sure.