feat: add Shiki syntax highlighting with theme-aware light/dark mode for code blocks

[j4rviscmd]

Summary

Add VS Code-quality syntax highlighting to code blocks in the editor using Shiki, with automatic light/dark theme switching.

Changes

• Shiki highlighter bundle: Added precompiled bundle with 27 languages and light-plus/dark-plus themes via shiki-codegen for optimal loading performance
• Dual-theme parser injection: Intercept BlockNote's global Shiki parser cache to enable CSS variable-based dual-theme output (color for light, --shiki-dark for dark)
• Theme-aware CSS overrides: Override BlockNote's hardcoded #161616 code block background with OKLCH theme-responsive colors; activate --shiki-dark / --shiki-dark-bg CSS variables in dark mode
• Language selector visibility fix: Override hardcoded white text color on the language selector so it remains visible in light mode

Supported Languages (27)

JavaScript, TypeScript, JSX, TSX, HTML, CSS, JSON, Python, Rust, Bash, SQL, YAML, Markdown, Go, Java, C, C++, Ruby, PHP, Swift, Kotlin, Lua, Diff, TOML, XML, Dockerfile, GraphQL

How It Works

BlockNote internally uses prosemirror-highlight/shiki which calls codeToTokens with only the first loaded theme. By pre-populating globalThis[Symbol.for("blocknote.shikiParser")] with a dual-theme parser (using Shiki's themes option), each token receives both light-mode color and a --shiki-dark CSS variable. A companion CSS rule in index.css activates the dark palette when .dark is present on the document root.