function getUTF16Length(str, pos) {
  return 1 + isCodeUnitInSurrogateRange(str.charCodeAt(pos));
}
Return the length of the original Unicode character at given position in the
String by looking into the UTF-16 code unit; that is equal to 1 for any
non-surrogate characters in BMP ([U+0000..U+D7FF] and [U+E000, U+FFFF]); and
returns 2 for the hi/low surrogates ([U+D800..U+DFFF]), which are in fact
representing non-BMP characters ([U+10000..U+10FFFF]).

Examples:
- '\u0020' => 1
- '\u3020' => 1
- '\uD835' => 2
- '\uD835\uDDEF' => 2
- '\uDDEF' => 2

@param {string} str  Non-empty string
@param {number} pos  Position in the string to look for one code unit
@return {number}      Number 1 or 2