feat(menu): add Mermaid diagram entry to Blocks menu

[xhon-pelushi]

Summary

• Adds a "Mermaid diagram" item to the Blocks menu so inserting Mermaid is obvious.
• Outside a code block it inserts a Mermaid code block; inside a code block it sets the language to `mermaid`.

Fixes #8232

Test plan

• Open Text editor → Blocks menu → Mermaid diagram inserts a Mermaid code block
• Inside an existing code block, choosing Mermaid switches the language to mermaid
• `npm test` / menu entry unit tests pass

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