add WIP lua binding app

include/twn_util.h

@@ -67,6 +67,8 @@ TWN_API int64_t file_to_bytes(const char *path, unsigned char **buf_out);
 /* returns a pointer to a string which must be freed */
 TWN_API char *file_to_str(const char *path);
 
+/* returns true if the file exists in the filesystem */
+TWN_API bool file_exists(const char *path);
 
 /* saves all texture atlases as BMP files in the write directory */
 TWN_API void textures_dump_atlases(void);