Add TextEncoder and TextDecoder support

[oleiade]

What?

Add support for text encoding/decoding operations via a new k6/experimental/encoding module. This brings the WHATWG Encoding API standard to k6, implementing the TextEncoder and TextDecoder interfaces for converting between strings and byte arrays.

This code is imported from https://github.com/oleiade/xk6-encoding with modifications to integrate it as a built-in experimental module.

Key Changes

• New module: k6/experimental/encoding with TextEncoder and TextDecoder classes
• Supported encodings: UTF-8, UTF-16LE, UTF-16BE
• Features:
  • UTF-8 encoding via TextEncoder
  • Multi-encoding decoding via TextDecoder with configurable error handling
  • Streaming support for chunked data processing
  • BOM (Byte Order Mark) handling
  • Fatal and non-fatal decoding modes
• Dependencies: Adds golang.org/x/text v0.31.0
• Tests: Comprehensive test suite based on Web Platform Tests (WPT)
• Example: Added usage example in examples/experimental/encoding.js

Usage example

  import encoding from 'k6/experimental/encoding';

  // Encoding
  const encoder = new encoding.TextEncoder();
  const encoded = encoder.encode('Hello, 世界! 🌍');
  console.log(encoded); // Uint8Array with UTF-8 bytes

  // Decoding
  const decoder = new encoding.TextDecoder('utf-8');
  const decoded = decoder.decode(encoded);
  console.log(decoded); // "Hello, 世界! 🌍"

  // Streaming decoding
  const streamDecoder = new encoding.TextDecoder('utf-8');
  const chunk1 = streamDecoder.decode(part1, { stream: true });
  const chunk2 = streamDecoder.decode(part2, { stream: true });
  const final = streamDecoder.decode(part3); // stream: false (default)

Implementation limitations

As documented in the original xk6-encoding extension:

1. Limited encoding support: Only UTF-8, UTF-16LE, and UTF-16BE are implemented. Legacy encodings from the Encoding Living Standard (ISO-8859-*, Windows code pages, etc.) are intentionally out of scope.
2. API surface:
   - TextEncoder always outputs UTF-8 (per spec)
   - Streaming helper interfaces like TextDecoderStream are not exposed; use repeated decode() calls with {stream: true} instead
3. Fatal mode: When fatal: true is set, the decoder throws a TypeError on the first invalid byte sequence

Must do before going out of draft

• Automate Web Platform Tests import, in the same fashion as what's done for webcrypto.

Checklist

• I have performed a self-review of my code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have added tests for my changes.
• I have run linter and tests locally (make check) and all pass.
• I have added the correct milestone and labels to the PR.
• I have updated the release notes: link
• I have updated or added an issue to the k6-documentation: grafana/k6-docs#NUMBER if applicable
• I have updated or added an issue to the TypeScript definitions: grafana/k6-DefinitelyTyped#NUMBER if applicable

Related PR(s)/Issue(s)

closes #2440