do not truncate usernames by word boundaries, instead truncate by grapheme

[kaizoplant]

Previously, usernames truncated by user_gutter_width with word boundaries, such as display names like [somelongtexthere] or more relevant, usernames like @username:some.domain would be completely cut off after the first character. This pr makes it so that these will always just use the grapheme calculation directly.

Maybe this should be a config option? Although I do think it is usually preferable to have the username as visible as possible, it might still be useful to not always have this behavior.

[VAWVAW]

I believe the UnicodeSegmentation functions were always relatively CPU intensive. Have you tested whether this may introduce lag on lower-end systems?

[kaizoplant]

It doesn't seem to show up on the flamegraph for me so I imagine it shouldn't be too much of a problem. Also previously, it was still calling into split_word_bound_indices which I imagine was a bit worse for long usernames and display names (especially display names given it'd have to call split_word_bound_indices and then grapheme_indices after it failed), maybe it's better if it calls into something else entirely but it doesn't seem like too much of an issue rn?

[VAWVAW]

I see.