clang-format: [JS] FormatToken.startsSequence/endsSequence.

Refactors AnnotatedLine.startsWith/endsWith by extracting the core functionality
into FormatToken.startsSequence/endsSequence. This allows checking tokens within
the pointered linked list structure with a lookahead, automatically ignoring
comments, which is useful in many places (e.g. see subsequent commit).

llvm-svn: 271183