fix(gatsby-transformer-remark): fix plugin conflict causing escaped HTML in headings

[Ameobea]

Description

• The gatsby-remark-prismjs plugin was causing HTML code to be escaped and rendered as text in TOC entries generated by gatsby-transformer-remark. It generated HTML nodes which the mdast-util-toc module flattened into text. This was fixed upstream in a major version bump.
• Update the version of mdast-util-toc used in gatsby-transformer-remark to the latest version in order to fix the issue
  • Set { allowDangerousHTML: true } on calls to hastToHTML() and toHAST() when building the TOC. This is required in order to properly render the embedded HTML nodes.
• Bump mdast-util-to-hast to its latest version as well
  • Update one snapshot test that was very slightly broken as a result of this
• Slight refactor of code in on-node-create.js to reduce nesting and clean up control flow
• Add two snapshot tests to verify that the fix was successful

Related Issues

Fixes #13608

Additional Info

This does not address updating the generated anchor link as mentioned here: #13608 (comment)

Ideally, another change would be applied to remove the HTML artifacts from the anchor link.

[pieh]

I just merged few other remark PRs that cause merge conflicts here - happy to resolve them if you want me to (don't want to push to your branch without asking first :) )

[Ameobea]

I've taken feedback into account and rebased to the latest master. Feel free to squash these two commits when merging to produce a cleaner history.

[Ameobea]

Hey - just wanted to bump this since it's close to getting off the first page of PRs and I know this is an incredibly active repo.

I've made all requested changes as per code review and reduced the size of the diff. It's been freshly rebased to the latest master.

[pieh]

Let's get this in. Thanks for research, fixes and tests!

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