JavaScript editor misses syntax highlighting for multi line template strings

According to `https://developer.mozilla.org/de/docs/Web/JavaScript/Reference/template_strings` multiline template strings are valid. Qt Creator JS Editor does not highlight these strings.

I would expect the JS Editor highlights the template strings as also variable references insight these template strings.

this works
```
var tpl = `hello ${hello}`
```

this fails
```
var tpl = `
hello ${hello}
`
```

The closing ``` is not recognised and all text beyond is coloured as comment text.