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.
Specifically, do not allow multiline single-quote strings, which are not a
thing in Julia. The existing rule broke when adjoints were used, such as
`b = a'`.
The syntax rules have been copied from Rust, which also uses single ticks for
character literals, and also uses the ' symbol for things unrelated to chars.
