fix: disable syntax highlighting for code blocks without language specifier

[codeananda]

Summary

Code blocks without a language specifier (just triple backticks) were getting unwanted syntax highlighting due to highlight.js auto-language detection. Words like "NATURAL", "Type", "OR", "trigger", "within" were being coloured as if they were programming keywords.

This was likely worsened by the recent dependency upgrade (commit 65e2616).

Changes

• Added check for empty lang in the fence renderer to skip highlight.js processing
• When no language is specified, renders as plain <pre><code> without hljs classes
• Added vitest testing infrastructure with comprehensive tests

Test plan

• Code blocks without language specifier render as plain text (no syntax highlighting)
• Code blocks WITH language specifier still get syntax highlighting
• Mermaid blocks still render correctly as diagrams
• HTML is properly escaped in plain code blocks
• All 7 tests pass (yarn test)
• Lint passes (yarn lint)
• Build succeeds (yarn build)
• Manual testing in MarkEdit confirms fix works

[codeananda]

Before fix example

After fix example

[cyanzhong]

Thank you for submitting the PR.

While the change serves its purpose, I believe we should address the following points:

• There's a recommended approach for disabling auto-detection: markdown-it-highlightjs. I think we should take advantage of it.
• While we're unsure whether auto-detect is optimal, it seems to be the default option for a reason. Perhaps we could introduce a configurable option for this behavior.
• The change isn't fully atomic, as it also introduces unit test infrastructure.

To resolve these, would you be open to merging this PR and repurposing your PR to focus solely on adding the tests?

Looking forward to hearing your thoughts.