Allow completions for arbitrary strings

There are many use cases where path completions are desired which are not in require/import statements, e.g.

```js
@Component({
...
templateUrl: './please-complete-me.html',
})
```

Allowing completions in the above case and others would be very useful. Depending on the performance impact it could be enabled/disabled through an option.

Start by reviewing the existing path-completion behavior for require/import statements and compare it with the JavaScript Component templateUrl example in the issue. Define which arbitrary string contexts are supported, measure the performance impact, and make the behavior configurable if needed; done means the example and agreed additional contexts receive correct completions without unacceptable slowdown.

Written by the indexing model from the issue text.