syntaxes: use one regex for coloring quoted strings, to avoid overlap

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.

syntax/javascript.nanorc

@@ -17,7 +17,7 @@ color cyan "\<([0-9]+|0x[0-9A-Fa-f]+)\>"
 color cyan "\<(true|false|null|undefined)\>"
 
 # Strings.
-color brightmagenta ""(\\.|[^"])*"" "'(\\.|[^'])*'" "`(\\.|[^`])*`"
+color brightmagenta ""(\\.|[^"])*"|'(\\.|[^'])*'|`(\\.|[^`])*`"
 # Comments.
 color brightblue "(^|[[:blank:]])//.*"
 color brightblue start="/\*" end="\*/"