Allow autocompletion when using +=

[ModProg]

This allows to use the autocompletion when concatinating strings using
+=.

This allows to use autocompletion when e.g. writing conditional styling
this way:

var className = "text-red-50";
if (shouldBeRounded){
    className += "rounded";
}

[bradlc]

Please see #465 (comment)

[ModProg]

For anyone coming across this needing this as regex you can use:

'(?:classes|class)\\s*\\+?=\\s*"([^"]*)',
'(?:classes|class)\\s*\\+?=\\s*`([^`]*)',
"(?:classes|class)\\s*\\+?=\\s*'([^']*)",

Replace classes|class with all the variable names you want to use.

Still would have considered this a small enough change to be added in the code IMO.