Quoted strings cannot start within another quoted string and end after
that other string has ended. Therefore single-quoted and double-quoted
strings should (as much as possible) be colorized by a single rule, so
that overlapping colorations are avoided.
(This also fixes a double typo in the PHP syntax (\. --> \\.) that has
been there since the PHP syntax was added in 2008, commit 90ee8ee4.)
This fixes https://savannah.gnu.org/bugs/?61387.
Bug existed since each of these syntaxes was introduced,
the oldest ones around 2006, a few others around 2015.
The one that got it right was the Lua syntax from 2011.
In many places a carriage return is not valid whitespace and should
thus not be colored as such. In some of these places a vertical tab
or form feed is maybe valid whitespace, but it would be ugly or even
wrong to color them because they are not part of the subsequent
comment or keyword.
This fixes https://savannah.gnu.org/bugs/?60456.
