Add is_(leading|trailing)_surrogate/1 to char

For input validation and character encoding transformations
it is essential to check for surrogate characters and their correct
sequence.

NEWS:
    Announce the addition of is_(leading|trailing)_surrogate/1.

library/char.m:
    Add is_leading_surrogate/1 which succeeds if a character is a
    leading surrogate character.
    is_trailing_surrogate/1 succeeds if the character is a
    trailing surrogate character.