Add builtins highlighting

test/highlight/builtins.bp

@@ -0,0 +1,14 @@
+foo = select(soong_config_variable("my_namespace", "my_var"), {
+    //       ^ function.builtin
+    "foo": unset,
+    //     ^ variable.builtin
+    default: select(variant("VARIANT") {}),
+    // <- variable.builtin
+    //              ^ function.builtin
+})
+
+/* Assigning to builtins is conveniently not allowed at runtime */
+unset = 12
+// <- variable.builtin
+default = 27
+// <- variable.builtin