Highlight \ escapes in " strings conspicuously

[lheckemann]

The only cases where a backslash should usually be found in a double-quote string are before \, ${, n and t. Any other use of \ in a double-quote string will swallow the backslash, and is likely unintentional. A common case is in regexes: nix "\." is equivalent to ".", though that's not likely to be what an expression author means! Thus, I'd suggest highlighting any such backslashes in a conspicuous fashion.