Add support for Unity versions earlier than 2022

[kafeijao]

This change allows me to use this package on Unity 2021

The unity editor automatically sets UNITY_2022_1_OR_NEWER to true

This fix was also mentioned here: #30 (comment)

[kafeijao]

In 5ddece0 I added the missing meta file to prevent unity throwing the error:

This didn't stop it from working, but having the error pop every now and there is eh

Edit: I noticed that particular file was on the .gitignore, so I reverted. I assume it's being ignore for a reason and me getting the error is probably related with me using a lower version of unity (2021)

[PiMaker]

The unitypackage.list file is for the old way of automatically building unitypackages, it's harmless and not included when loading via the VCC.

Thanks for the PR!