Add `std::encoding` module (json, base64, hex, url, utf8)

[mjm918]

Summary

Add a comprehensive std::encoding module for common encoding/decoding operations.

Sub-issues

This feature has been split into smaller, focused issues:

• Add std::encoding::base64 module #50 - std::encoding::base64 - Base64 encoding/decoding
• Add std::encoding::hex module #51 - std::encoding::hex - Hexadecimal encoding/decoding
• Add std::encoding::url module #52 - std::encoding::url - URL percent-encoding/decoding
• Add std::encoding::utf8 module #53 - std::encoding::utf8 - UTF-8 encoding/decoding
• Add std::encoding::json module #54 - std::encoding::json - JSON encoding/decoding with dynamic json type

Module Structure

std::encoding::base64   (#50)
std::encoding::hex      (#51)
std::encoding::url      (#52)
std::encoding::utf8     (#53)
std::encoding::json     (#54)

Implementation Order

Recommended order (simplest to most complex):

1. utf8 (Add std::encoding::utf8 module #53) - Simple, strings are already UTF-8 internally
2. hex (Add std::encoding::hex module #51) - Straightforward byte<->string conversion
3. base64 (Add std::encoding::base64 module #50) - Well-defined standard encoding
4. url (Add std::encoding::url module #52) - Percent-encoding with some edge cases
5. json (Add std::encoding::json module #54) - Most complex, requires new type system support

Shared Exception

All modules share:

exception DecodeError {
    position: int,
}

Completion Criteria

This tracking issue will be closed when all sub-issues are completed.