feat(buffer): add encoding support and shared TypedArray prototype

• Add Buffer.from/toString encoding support for base64, hex, utf8, utf-8, ascii, latin1, binary, ucs2, ucs-2, utf16le, utf-16le (case-insensitive)
• Add Buffer static methods: isBuffer, isEncoding, byteLength, concat, compare
• Create shared TypedArray.prototype with slice, subarray, fill methods inherited by all typed array prototypes
• Fix Buffer to use proper constructor pattern with prototype chain
• Fix Object.prototype.toString to follow prototype chain for Symbol.toStringTag lookup
• Simplify Buffer.isBuffer to compare prototypes directly
• Remove redundant per-instance method assignments