CLN: preliminary refactor before `Styler.highlight_quantile`
The current structure is:

```
def highlight_between:
    def f:
        return x
    return self.apply(f)
```

So that `Styler.highlight_quantile` can reuse code this PR refactors the above to:

```
def highlight_between:
    return self.apply(_new_name)

def _new_name:
    return x
```