Add Grapheme cluster support

[adammw]

No description provided.

[liggitt]

I'm in favor of fixing the width counting issue but not in favor of picking up a dependency outside the stdlib (or maaaaaybe golang.org/x/text). It looks like this was proposed to be upstreamed into golang.org/x/text in golang/go#14820 (comment).

I'd accept a change that used an upstreamed version of this in golang.org/x/text or unicode/utf8, or a direct implementation of the Grapheme Cluster Boundary Rules in a private function in this library.

[adammw]

Updated PR to attempt to use a direct implementation from the uniseg code but there's an awful lot of lookup tables that are required to implement them, and the golang.org/x/text issue does not seem to have progressed since 2019.

[adammw]

This has become a rabbit hole.. most terminals seem to support Unicode 8 or 9 rather than the latest and treat most Emoji that should be double-width as single width, with additional advanced settings to only make them double-width in the presence of Emoji-presentation variation selectors. Therefore I'm not sure a standard approach can be reached without numerous flags to support all options.