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/fzl_parser.h

@@ -3,6 +3,7 @@
 
 #include <compiler/fzl_reader.h>
 #include <compiler/fzl_token.h>
+#include <stdbool.h>
 #include <stddef.h>
 
 // AST node type
@@ -106,6 +107,7 @@ typedef struct fzl_node
     {
       fzl_token_t *type;
       struct fzl_node *identifier;
+      bool is_out;
     } var;
 
     // Conditional blocks

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