Previously, we were mistakenly adding `tab_size` to the extent
produced by the fold edits but that could cause the edit to land on
a multi-byte character (like a fold or an emoji).

In practice, we only need to expand the edit's extent by 1 because
we are operating in the fold coordinate space and all we need to
convey is that we want to encapsulate the first tab next to whatever
edit has just occurred in the `FoldMap`.