fix(ansi): truncate: truncate string when input length is equal to output

[aymanbagabas]

goos: darwin
goarch: arm64
pkg: github.com/charmbracelet/x/ansi
                  │   old.txt   │              new.txt               │
                  │   sec/op    │   sec/op     vs base               │
TruncateString-16   24.96n ± 1%   25.83n ± 2%  +3.49% (p=0.001 n=10)

                  │  old.txt   │            new.txt             │
                  │    B/op    │    B/op     vs base            │
TruncateString-16   65.00 ± 0%   65.00 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal

                  │  old.txt   │            new.txt             │
                  │ allocs/op  │ allocs/op   vs base            │
TruncateString-16   1.000 ± 0%   1.000 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal

Fixes: charmbracelet/lipgloss#324
Fixes: #106

CC/ @mikelorant @jbcpollak

[aymanbagabas]

k, was able to get nearly the same performance by improving the implementation of StringWidth