is_known_utf8 check taken from into_string

Author: tnuha

library/std/src/sys_common/wtf8.rs

@@ -480,6 +480,7 @@ impl Wtf8Buf {
     #[inline]
     pub(crate) fn extend_from_slice(&mut self, other: &[u8]) {
         self.bytes.extend_from_slice(other);
+        self.is_known_utf8 = self.is_known_utf8 || self.next_surrogate(0).is_none();
     }
 }