Move mjz_decoder out of experimental to xml/mjz.

Since the mjz format is just an archive for MJCF assets this seems like a sensible place. The upcoming mjz encoder will also need to make use of the full precision XML utility that is local to src/xml.

PiperOrigin-RevId: 912044746
Change-Id: I6a9fef24b1c3fec5b5edc3a8d4c8e22584273264

test/xml/mjz/CMakeLists.txt

@@ -0,0 +1,19 @@
+# Copyright 2021 DeepMind Technologies Limited
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+mujoco_test(mjz_api_test)
+
+mujoco_test(
+  mjz_decoder_test
+)