build: Generate headings with anchors

[juliaogris]

Generate headings with anchors with md tool, in the same way GitHub creates
anchors on .md pages. Use the same style of IDs: heading text with all
non-letter/numbers replaced by single dash.

In preparatory step rework walk function in the Markdown generation. We now
allow for nodes not only to be modified but also to be replaced or deleted.

For what we are trying to achieve here this might be over-kill, but it is
preparatory work for building a multiple-choice test generator.

It seems more correct to me to add a new type of node called anchoredHeading,
as it seems to be a different thing to an ordinary heading. in the markdown
page, Tasks are also explicitly modeled when used instead of unordered
lists.

I'll put up an alternative PR that is just straight-forward overriding the
markdown.Heading.PrintHTML function for comparison.

Link: https://stackoverflow.com/questions/43273842/what-are-the-rules-of-converting-one-markdown-title-into-an-html-anchor/44443100#44443100
Issue: evylang/todo#61

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Fixes evylang/todo#61

[github-actions]

firebase-deployment for 5785632

• apex: https://evy-lang-stage--270-5d2j3p42.web.app
• discord: https://evy-lang-stage-discord--270-kexvdpaf.web.app
• docs: https://evy-lang-stage-docs--270-v5jisbn9.web.app
• learn: https://evy-lang-stage-learn--270-cekwpm0m.web.app
• play: https://evy-lang-stage-play--270-2pqsehg0.web.app

[juliaogris]

Closing this as it seems too complicated for what we are trying to achieve here in favor of #272 .