REF: type change `escape` in `Styler.format` to str to allow "html" and "latex"
Change is made ahead of 1.3.0 to pre-empt the addition of `escape_latex` and to avoid name ambiguity.

This is not user facing since the `escape` kwarg was added for 1.3.0 only recently.

**Alternative**

If instead of having separate `escape_html` and `escape_latex` kwargs we could have `escape={None, 'html', 'latex'}` since it is likely they will be used exclusively??