function isLowSurrogate(codePoint) {
    return codePoint >= LOW_SURROGATE_START && codePoint <= LOW_SURROGATE_END;
  }
Checks if `codePoint` is a low-surrogate number from range 0xDC00 to 0xDFFF.

@ignore
@param {number} codePoint The code point number to be verified
@return {boolean} Returns a boolean whether `codePoint` is a low-surrogate number.