Enable json v2 support for 59% faster sourcemap parsing

[pedro-stanaka]

Summary

This PR adds support for the experimental encoding/json/v2 package through build tags, providing significant performance improvements for sourcemap parsing while maintaining full backward compatibility.

Changes

• Add unmarshalJSON() abstraction layer to switch between JSON implementations
• Create json_decoder.go for standard encoding/json (default)
• Create json_decoder_v2.go with jsonv2 build tag for Go 1.25+
• Replace 2 json.Unmarshal calls with the abstraction in consumer.go
• Minor bonus: Use strings.Builder for more efficient URL concatenation

Performance Improvements

Benchmarks show 50-74% faster parsing across different sourcemap sizes:

Sourcemap Size	Standard JSON	JSON/v2	Improvement
Small (5KB)	66 MB/s	115 MB/s	+74%
Medium (100KB)	104 MB/s	176 MB/s	+69%
Large (500KB)	113 MB/s	180 MB/s	+59%
XLarge (2.5MB)	125 MB/s	189 MB/s	+51%
XXLarge (10MB)	118 MB/s	187 MB/s	+59%

Key Metrics

• Throughput: 118 MB/s → 187 MB/s (59% improvement on large files)
• Allocations: 444 → 413 (7% fewer)
• Memory usage: Roughly the same

Usage

The change is opt-in and requires no code changes from users:

# Standard build (default - uses encoding/json)
go build ./...

# With json/v2 optimization (Go 1.25+)
GOEXPERIMENT=jsonv2 go build -tags=jsonv2 ./...

Benchmarks

I have comprehensive benchmarks that demonstrate these improvements across various sourcemap sizes (from 5KB to 10MB). If you're interested, I can submit them as a follow-up/preamble PR. The benchmarks include:

• Synthetic sourcemap generator for consistent testing
• Performance tests across 6 different file sizes
• Memory allocation analysis
• Throughput measurements in MB/s

Why This Matters

Many modern web applications produce large sourcemaps (2-20MB is common), and parsing performance directly impacts:

• Build tool performance
• Error reporting services
• Development server response times
• CI/CD pipeline speeds

This simple change provides substantial performance improvements with zero risk to existing users.

References

• encoding/json/v2 Proposal
• JSON v2 Experimental Release

[vmihailenco]

Looks good, thanks 👍