go-libghostty

Add human-friendly string conversion to the input enum types so they can be serialized to and parsed from text formats like JSON or configuration files. Each type gains a String() method returning the canonical snake_case name (e.g. "key_a", "arrow_down", "shift+ctrl", "middle", "gained") and a pointer-receiver FromString method that mutates the receiver. A top-level NewKeyFromString, NewModsFromString, NewMouseButtonFromString, and NewFocusEventFromString constructor is also provided as a more ergonomic alternative.

This brings in (generated) wrappers for the paste helpers.

Fixes #6 The root libghostty package previously imported image/png for SysDecodePng. Because image/png registers itself via an init() function, the Go linker cannot eliminate it from binaries that never call SysDecodePng. Every consumer paid the cost. Move the built-in PNG decoder into sys/png so users opt in explicitly. The root package no longer imports image or image/png. The kitty_graphics_test.go file inlines a local test helper to avoid the import cycle that would result from an internal test package importing its own subpackage.