Replace MurmurHash with Rust's DefaultHasher for CSS identifiers

[jantimon]

Replace our custom MurmurHash2 implementation with Rust's built-in DefaultHasher (SipHash-1-3) for generating CSS identifiers

Key Changes:

• Replace murmur_hash.rs with css_hash.rs using Rust's DefaultHasher
• Generate base62 instead of base36 hashes to allow more characters in identifiers
• Always start generated identifiers with a letter to ensure CSS validity
• Update hash length for better compatibility and readability

For example, hashes now look like:

- y_17k2ec6  // old format (base36)
+ yupb5bt    // new format (base62, starts with letter)

This change prepares us for implementing unique class name generation once Next.js adds support for global styles inside PostCSS CSS module files