libghostty: add log callback configuration (#12227)

In C ABI builds, the Zig std.log default writes to stderr which is not
appropriate for a library. Override std_options.logFn with a custom sink
that dispatches to an embedder-provided callback, or silently discards
when none is registered.

Add GHOSTTY_SYS_OPT_LOG to ghostty_sys_set() following the existing
decode_png pattern. The callback receives the log level as a
GhosttySysLogLevel enum, scope and message as separate byte slices,
giving embedders full control over formatting and routing.

Export ghostty_sys_log_stderr as a built-in convenience callback that
writes to stderr using std.debug.lockStderrWriter for thread-safe
output. Embedders who want the old behavior can install it at startup
with a single ghostty_sys_set call.