String escapes like `''\${1+x}` should not be touched

[cormacrelf]

For example

bashString = ''
    if [[ -z ''\${1+x} ]]; then echo "no arg supplied"; fi
'';

Is currently reformatted to

bashString = ''
    if [[ -z ''\${1 + x} ]]; then echo "no arg supplied"; fi
'';

which is invalid bash. (${1+x} is a modifier a bit like ${1:-}). The correct formatting is to leave it exactly as-is because it's not a ${} substitution with a nix expression inside, the $ is escaped.