Disallow import declarations within a when statement

src/parser.hpp

@@ -116,6 +116,7 @@ struct AstFile {
 	bool         allow_in_expr; // NOTE(bill): in expression are only allowed in certain cases
 	bool         in_foreign_block;
 	bool         allow_type;
+	bool         in_when_statement;
 
 	isize total_file_decl_count;
 	isize delayed_decl_count;