feat(meta/widget): Add css classes for indentation as required by group and nest. (#3764)
feat(meta/widget): Add css classes for indentation as required by group and nest. (#3764)

this is a transplant of https://github.com/leanprover-community/lean/pull/439

the relevant css section looks more or less like this:
```css
        .indent-code {
            text-indent: calc(var(--indent-level) * -1ch);
            padding-left: calc(var(--indent-level) * 1ch);
        }
```

For details, one can play around here: https://jsfiddle.net/xbzhL60m/45/


Co-authored-by: Ed Ayers <5525be1fdf6221ee30be52359259248ffc35fe1a@users.noreply.github.com>
Co-authored-by: Gabriel Ebner <2ee8ec943467fad83db2eb3f247bb99e61ef3d05@gebner.org>
Co-authored-by: E.W.Ayers <1de7ee4668bc5f8939bb069fd3095734a7ef172f@maths.cam.ac.uk>

leanprover-community/mathlib,leanprover-community/mathlib