tui: only check for emojis in visibleWidth when necessary

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

package-lock.json

@@ -5529,22 +5529,6 @@
 				"safe-buffer": "~5.2.0"
 			}
 		},
-		"node_modules/string-width": {
-			"version": "8.1.0",
-			"resolved": "https://registry.npmjs.org/string-width/-/string-width-8.1.0.tgz",
-			"integrity": "sha512-Kxl3KJGb/gxkaUMOjRsQ8IrXiGW75O4E3RPjFIINOVH8AMl2SQ/yWdTzWwF3FevIX9LcMAjJW+GRwAlAbTSXdg==",
-			"license": "MIT",
-			"dependencies": {
-				"get-east-asian-width": "^1.3.0",
-				"strip-ansi": "^7.1.0"
-			},
-			"engines": {
-				"node": ">=20"
-			},
-			"funding": {
-				"url": "https://github.com/sponsors/sindresorhus"
-			}
-		},
 		"node_modules/string-width-cjs": {
 			"name": "string-width",
 			"version": "4.2.3",
@@ -6614,9 +6598,9 @@
 			"dependencies": {
 				"@types/mime-types": "^2.1.4",
 				"chalk": "^5.5.0",
+				"get-east-asian-width": "^1.3.0",
 				"marked": "^15.0.12",
-				"mime-types": "^3.0.1",
-				"string-width": "^8.1.0"
+				"mime-types": "^3.0.1"
 			},
 			"devDependencies": {
 				"@xterm/headless": "^5.5.0",