lang: ast: move block parser state to separate header file

this allows other parsers to specify what kind of token should end the
block.

lang/ast/block.h

@@ -0,0 +1,12 @@
+#ifndef _AST_BLOCK_H_
+#define _AST_BLOCK_H_
+
+#include "ctx.h"
+
+struct block_parser_state {
+	struct parser_state s_base;
+	bool s_single_expr;
+	unsigned int s_terminator;
+};
+
+#endif