Initial tree-sitter grammar for Dgraph DQL

Skeleton grammar supporting blocks, comments, strings, numbers,
and identifiers. Full DQL keyword support planned for Phase 2.

examples/simple.dql

@@ -0,0 +1,8 @@
+# A simple block structure
+myquery {
+  name
+  age
+  friend {
+    name
+  }
+}