vim-patch:8.2.3522: cannot use \x and \u when setting 'listchars' (#16049)

Problem:    Cannot use \x and \u when setting 'listchars'.
Solution:   Support hex and unicode in hex form. (closes vim/vim#9006)
93ff6720fe

src/nvim/charset.c

@@ -1642,6 +1642,16 @@ int hex2nr(int c)
   return c - '0';
 }
 
+/// Convert two hex characters to a byte.
+/// Return -1 if one of the characters is not hex.
+int hexhex2nr(char_u *p)
+{
+  if (!ascii_isxdigit(p[0]) || !ascii_isxdigit(p[1])) {
+    return -1;
+  }
+  return (hex2nr(p[0]) << 4) + hex2nr(p[1]);
+}
+
 /// Check that "str" starts with a backslash that should be removed.
 /// For Windows this is only done when the character after the
 /// backslash is not a normal file name character.