Refactor parser header

Make separate lexer, stack and parser structs.

src/runtime/parser.c

@@ -0,0 +1,10 @@
+#include "tree_sitter/runtime.h"
+
+const ts_tree * ts_parser_parse(ts_parser *parser, ts_input input) {
+    return parser->parse_fn(parser->data, input);
+}
+
+void ts_parser_free(ts_parser *parser) {
+    if (parser->free_fn != NULL)
+        parser->free_fn(parser->data);
+}