wip: working on extern

- Ref: move extern header into common scope
- Feat: add extern symbols descriptors
- Feat: add `is_out` flag in declaration node

include/compiler/utils/fzl_extern.h

@@ -1,16 +0,0 @@
-#ifndef FZL_EXTERN_H
-#define FZL_EXTERN_H
-
-// Library symbol descriptor
-typedef struct
-{
-  //
-} fzl_extern_item_t;
-
-// External library descriptor
-typedef struct
-{
-  //
-} fzl_extern_lib_t;
-
-#endif // FZL_EXTERN_H