Turning `header` patterns into `signature` patterns in all syntax files
was a mistake. The two are different things. In almost all syntax files
those patterns are things like shebangs or <?xml ... ?> or
<!DOCTYPE html5> i.e. things that:
1. can be (and should be) used for detecting the filetype when there is
no `filename` match (and that is actually the purpose of those
patterns, so it's a regression that it doesn't work anymore).
2. should only occur in the first line of the file, not in the first
100 lines or so.
In other words, the old `header` semantics was exactly what was needed
for those filetypes, while the new `signature` semantics makes little
sense for them.
So replace `signature` back with `header` in most syntax files. Keep
`signature` only in C++ and Objective-C syntax files, for which it was
actually introduced.
Use the 'regexp2' library for lookahead and lookbehind in region
start and end regular expressions to support things like closing quotes
that aren't preceded by backslashes.
