Fix compatiblity with Cloudflare - #463

This fixes Cloudflare compat, can fully use this libary in dev mode with Workerd.

High-level, the fix works because:

• Uses Shiki’s workerd-compatible Oniguruma WASM entrypoint.
• Replaces PostCSS/PostCSS Nested with @eslint/css-tree. This is hte biggest change and accounts for most of the diff.
• Bundle hast-util-select into core so the CommonJS version doesn't need to be optimized.

Fixes #439