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

[sebgod]

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.

[sebgod]

As discussed in the mailing list

On a side note, I think the new syntax highlighting is in place, any comments welcome.

[PaulBone]

I'm not a user of those editors and assuming that that's the new github syntax highlighting (I know it changed recently), I don't see github that way. Due to my vision impairment I see most webpages in white-on-black (firefox settings) so I don't have an oppinion at all. Other people might have noticed though.