build(rust): vendor rnix parser crate

third_party/crates/rnix-0.14.0/test_data/parser/success/interpolation.nix

@@ -0,0 +1,18 @@
+let
+  world = "World";
+in {
+  string = "Hello ${world}!";
+  multiline = ''
+    The set's x value is: ${
+      {
+        x = "1";
+        y = "2";
+      }.x
+    }
+
+    This line shall have no indention
+      This line shall be indented by 2
+    ''\n
+    ${ "hi" }
+  '';
+}