fix: move build tags to file beginning before copyright header

[knqyf263]

Summary

• Move build tags to the beginning of files, before copyright headers
• Add new //go:build syntax alongside legacy // +build syntax for compatibility

Problem

Build tags were placed after copyright headers, causing them to be ignored by the Go compiler. This resulted in Windows cross-compilation errors where both os_posix.go and os_windows.go were compiled together, leading to duplicate declarations:

GOOS=windows go build .
# github.com/joelanford/go-apidiff/pkg/diff/internal/osfs
pkg/diff/internal/osfs/os_windows.go:50:16: method file.Lock already declared at pkg/diff/internal/osfs/os_posix.go:29:16
pkg/diff/internal/osfs/os_windows.go:65:16: method file.Unlock already declared at pkg/diff/internal/osfs/os_posix.go:36:16
pkg/diff/internal/osfs/os_windows.go:77:6: rename redeclared in this block

Solution

According to Go's build tag specification, build constraints must be placed at the beginning of the file before any other comments. This fix moves the build tags to the proper location.