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

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+# A simple block structure
+myquery {
+  name
+  age
+  friend {
+    name
+  }
+}

examples/test.dql

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+# Simple DQL query
+{
+  me(func: eq(name, "Alice")) {
+    name
+    age
+    friend @filter(ge(age, 21)) {
+      name
+    }
+  }
+}
+
+# Mutation example
+{
+  set {
+    _:alice <name> "Alice" .
+    _:alice <age> "30" .
+  }
+}