Retain most HTML elements in heading label in Table of Contents

[seshrs]

Context

Fixes #160.

Here's how we generated heading labels in the Table of Contents before this PR: If a heading contained multiple HTML elements (for instance, formatting), we ignored most of those elements and only used text nodes and codeblocks. However, this was a mistake — @marcus-darden pointed out that headings can have italics and other formatting.

This PR updates the heading-label generation logic to preserve almost all formatting within headings (except for the AnchorJS link).

Validation

I updated the "Install the dependencies" heading in the Project 0 demo spec (index.md). Visit the preview URL and observe the label in the sidebar.

Preview URL: https://preview.sesh.rs/previews/eecs485staff/primer-spec/164/

Before	After

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Thanks for reporting @marcus-darden!

[github-actions]

The spec from this PR is available at https://preview.sesh.rs/previews/eecs485staff/primer-spec/164/.

^{(Available until Sat May 07 2022.)}