[os] Darwin: Add os.exists()

core/os/os_darwin.odin

@@ -473,6 +473,11 @@ is_dir_path :: proc(path: string, follow_links: bool = true) -> bool {
 is_file :: proc {is_file_path, is_file_handle}
 is_dir :: proc {is_dir_path, is_dir_handle}
 
+exists :: proc(path: string) -> bool {
+	cpath := strings.clone_to_cstring(path, context.temp_allocator)
+	res := _unix_access(cpath, O_RDONLY)
+	return res == 0
+}
 
 rename :: proc(old: string, new: string) -> bool {
 	old_cstr := strings.clone_to_cstring(old, context.temp_allocator)