add LL backend stub

src/backends/ll.c

@@ -0,0 +1,17 @@
+#include <assert.h>
+#include "../internal.h"
+#include "../parsers/parser_internal.h"
+
+
+int h_ll_compile(HAllocator* mm__, const HParser* parser, const void* params) {
+  return -1; // TODO
+}
+
+HParseResult *h_ll_parse(HAllocator* mm__, const HParser* parser, HParseState* parse_state) {
+  return NULL; // TODO
+}
+
+HParserBackendVTable h__ll_backend_vtable = {
+  .compile = h_ll_compile,
+  .parse = h_ll_parse
+};