chore: import upstream snapshot with attribution

ui/model/textinput.go

@@ -0,0 +1,12 @@
+package model
+
+import "unicode/utf8"
+
+// removeLastRune trims the final UTF-8 rune from s, used by all text input handlers.
+func removeLastRune(s string) string {
+	if len(s) > 0 {
+		_, size := utf8.DecodeLastRuneInString(s)
+		return s[:len(s)-size]
+	}
+	return s
+}