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.
485eb18d83