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 ⊭
```

Read the pinned issue first for contribution instructions, then find the dictionary data file containing the existing mathematical proof-symbol entries. Add the missing mappings for \nVDash (⊯) and \nVdash (⊮), and verify that the dictionary format remains consistent with the entries already added in issues #74, #75, #78, #83, and #85.

Written by the indexing model from the issue text.