basic check of js raw regex syntax

[bobzhang]

currently we have

let re = [%re "/a/" ]

It would be nice that we can do some basic checking to prevent bad things happen as below

let re = [%re "a" ]

which will be compiled into

var re = a

[bobzhang]

Note this could also be useful in #1124

This can be closed now I think

[bobzhang]

thanks