Add support for Unicode variation sequences

These are character pairs that should be treated as tokens with a
buffer delta of 2 and a screen delta of 1.
:memo: Mention variation sequence in specs
Add specs for variation sequences
:memo: Drop text
:memo: Mention variation sequence in comment
Don't treat consecutive variation selectors as a sequence
Return as soon as first paired character is found

Previously the character count of the entire string was taken even
though it was only checking for the presence of a paired character.

Now hasPairedCharacter returns as early as possible and the now
unused getCharacterCount has been removed.
:lipstick: Remove extra newline
Merge pull request #3565 from atom/ks-support-variation-sequences

Support variation sequences