This adds highlighting for
- quoting operators qw, qr, and qx, like `qw< some words >`
- angle brackets after a quoting operator, like `q<string>`
- punctuation as quoting delimiter, like `q|string|`
- POD sections, which start with ^=\w and and with ^=cut
- heredocs; the marker can be a bare word, or a quoted word, like
print <<~ 'EOF'
single quoted heredoc
EOF
Closes#3736
No attempt is made to use different highlighting for interpolated (qq or
"") strings just yet. Recognizing quote boundaries is more important.
This commit allows using the <semicolon> expansion in commands, instead
of `\;`.
It makes commands look more elegant, and prevents new-comers from
falling into the trap of using <a-;> without escaping the semicolon.
