adds a windows-specific method to check if a command is safe (#4119)

refactors command_safety files into its own package, so we can add
platform-specific ones
Also creates a windows-specific of `is_known_safe_command` that just
returns false always, since that is what happens today.

codex-rs/core/src/command_safety/mod.rs

@@ -0,0 +1,3 @@
+pub mod is_safe_command;
+#[cfg(target_os = "windows")]
+pub mod windows_safe_commands;