Deprecate Qml multiline string literals

Currently all string literals in Qml can contain newlines.
This was good when javascript had no support for multiline strings.
Now javascript has string templates

```

    greetings = "hi"
    s = `
       ${greetings}
       multiline
       template string`
```

and supporting newlines in the other strings is a needless source of incompatibility.

We should warn about them and support rewriting them as templates.

Please note that several javascript interpreters support newlines when preceded by a backslash, eating to some special handling of backslashes at the end of a multiline string in places.