fix: convert bin/next to JavaScript for Yarn PnP compatibility
Yarn Berry PnP doesn't support shell script binaries - it tries to parse
them as JavaScript. Convert bin/next from shell script to JavaScript with
#!/usr/bin/env node shebang.
The JS wrapper:
- Detects platform/arch
- Looks for Rust CLI binary in @next/cli-* packages
- Falls back to dist/bin/next if not found
Benchmark: ~25-35ms overhead vs shell script (negligible for 500-1500ms startup)
Also removes next.cmd as npm/yarn auto-generate .cmd wrappers for JS bins.
