Feature Request: add Lean syntax-highlighting support

[lakesare]

Description

Printing .lean files produces no syntax highlighting (or incorrect highlighting from auto-detection).
From what I see, this happens because Lean language grammar is not bundled with highlight.js core, it lives in the separate third-party package highlightjs-lean, maintained by the leanprover-community.

Steps to reproduce

1. Open any .lean file in VS Code with the vscode-lean4 extension installed
2. Print or preview with vsc-print
3. Observe: code is rendered as plain text with no syntax coloring

In the screenshot below, notice that e.g. comments don't get highlighted.

Expected behavior

Lean keywords, tactics, literals, and comments are syntax-highlighted, similar to other supported languages.

Suggested fix

Add highlightjs-lean as a dependency and register it:

const leanHljs = require('highlightjs-lean');
hljs.registerLanguage('lean', leanHljs);