fix: add a default sourceURL to aid debugging

[maartenbreddels]

Fixes #100
Now we get a better stacktrace:

Which we can click again, and get some context.

Note that also a user can add a sourceURL, e.g.:

        <template>
            <div @click="click">Clicked {{clicks}}</div>
        </template>
        <script>
            module.exports = {
                mounted() {
                    console.log('mounted');
                    a = window.a.b.c;
                    console.log('a', a);
                },
                methods: {
                    click() {
                        this.clicks += 1
                    }
                }

            }
            //# sourceURL=testtemplate.vue
        </script>

I can imagine that we'd want to add this based on the vue template filename, but that might leak sensitive information, so I didn't want to go there yet.