unity stand alone build support + unity editor mesh duplicate

[MyelinsheathXD]

1. stand alone unity build support . encapsulated all editor scripts with #if UNITY_EDITOR since mujoco namespace holding editor modules which cases to error during stanablone build

https://github.com/MyelinsheathXD/mujocoGameTools/commit/72618db6a9b1382ab7bdb3f37ce7ca8a22c4cdc6

2. On geometry duplicate, the visable mesh disappear in the editor. On duplicate phase in the editor , ExecuteinedinMode along with Update method removing original shared mesh on duplicate phase
   fixed by calling DestroyImmediate() only when Aplication is in Play mode

https://github.com/MyelinsheathXD/mujocoGameTools/commit/100ad5c91900b8c6d554f59151327164c06015df

the branch fork link https://github.com/MyelinsheathXD/mujocoGameTools/tree/v2.1.5fromComUnity

[google-cla[bot]]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[Balint-H]

Isn't addition 1 already addressed by https://github.com/deepmind/mujoco/commit/0180d839a9fa3c9dcb07d09f0b66697003bd71a6?