Add Anet ET4 configs

[thinkyhead]

Configurations for some Anet models.

[chepo92]

I was looking for this, @thinkyhead which environment should be used in VScode/PIO with this to compile?

[thisiskeithb]

@chepo92 The existing ET4 config and the ones in this PR don't have proper board or environments in the main Marlin repo yet.

[thisiskeithb]

I just submitted a draft PR that adds Anet ET4 and ET4P board support (these boards are used in the ET4 and ET5): MarlinFirmware/Marlin#20280

[Daniel-br0]

Keith, hello. I have all et series models, created user menu with useful gcodes for the printers, ajusted all movements, etc. I would like to contribute, I can test things and share what I have created. There are still few bugs, like blobs during printing of complex mesh. This part I can't fix, it is probably some CPU problem. I am writing first time on github, sorry if it is a wrong brach.