Mathematical logic proof symbols

Before submitting a PR, please read the pinned issue for instructions.

You may choose to add one or a few, to leave some for others to contribute.

A few missing symbols:

```py
\nVDash	⊯	
\nVdash	⊮	
```

Already added (#74, #75, #78, #83, #85):

```
\models	⊧	
\nequiv	≢	
\VDash	⊫	
\Vdash	⊩
\Vvdash	⊪	
\nvdash	⊬	
\vDash	⊨	
\nvDash	⊭	
```