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.
* highlighter: Fix regions and patterns inside regions
* highlighting: Remove 2nd recursive highlightRegion() call
...and add limitGroup checks to pattern search.
* yaml: Add TODO type highlighting
* highlighting: Don't stop in highlightRegion() at empty lines
...because possible region line end pattern must be detected.
* syntax/sh: Correct string handling due to additional pattern handling
* syntax/sh: Remove slash in variables
* highlighter: Accept nested region only in case it's within the current reagion
* highlighter: Accept nested patterns only in case it's within the current reagion
* highlighter: Don't search for nesting in case the region end was found at start
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.
