More robust Mermaid diagram fixing

[dividedmind]

This improves mermaid diagram fixing in the backend by:

• simplifying the code;
• implementing a hack to make dompurify work in the backend; this ensures broken diagrams are detected correctly (previously, when diagram validation attempted to call dompurify, the error would cause the validation to end prematurely and make the code ignore the spurious error and assume the diagram was valid even though an error could have been detected later) — while this does not solve @diagram when the diagram has unresolved errors, helpful information should be displayed #2071 entirely it at least makes the user see the broken code as text,
• show a message to the user when repairing a diagram so it doesn't look stuck.

[apotterri]

As long as you're here, can you take a look at upgrading to mermaid 10? I believe that would allow vscode-appland to link to a local appmap-js repo.

[dividedmind]

I actually did try, but mermaid 10 is ESM-only.

[appland-release]

🎉 This PR is included in version @appland/navie-v1.34.2 🎉

The release is available on:

• @appland/navie-v1.34.2
• GitHub release
• @appland/navie-v1.34.2

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