Typos reports typos in _typos.toml

Adding your own `extend-words` is quite problematic:

```
❯ cat _typos.toml 
[default.extend-words]
grey = "gray"

❯ typos
error: `grey` should be `gray`
  --> ./_typos.toml:2:1
  |
2 | grey = "gray"
  | ^^^^
```

I would suggest typos ignore its own configuration file when checking for typos