The initial render of a session, and any re-draws caused by terminal
resizing are noticeably slow, especially on conversations with 20+
turns and many tool calls.
From profiling with `bun --cpu-prof` (available since bun 1.3.2), the
majority of the rendering (90%) is spent on detection of emojis in the
string-width library, running the expensive `/\p{RGI_Emoji}$/v`
regular expression on every individual grapheme cluster in the entire
scrollback. I believe it essentially expands to a fixed search against
every possible emoji sequence, hence the amount of CPU time spent in it.
This change replaces the `stringWidth` from string-width with a
`graphemeWidth` function that performs a similar check, but avoids
running the `/\p{RGI_Emoji}$/v` regex for emoji detection unless it
contains codepoints that could be emojis.
The `visibleWidth` function also has two more optimisations:
- Short-circuits string length detection for strings that are entirely
printable ASCII characters
- Adds a cache for non-ASCII segments to avoid recomputing string length
when resizing
- Export wrapTextWithAnsi from tui package
- Add 'Handling Input' section to README with key detection example
- Document wrapTextWithAnsi in utilities section
