Use file_name_width in grid
Use file_name_width in grid

Filenames with characters that need multiple bytes in UTF-8, and/or
control characters like combining diacritics, would break the grid.
`StrExt::width` seems to do the right thing, and there's conveniently a
function here (which was otherwise unused) to call that.

ogham/exa,kbknapp/exa,mneumann/exa,rhysd/exa,kbknapp/exa,ogham/exa,rhysd/exa,mneumann/exa