Helper allows you to set `this.args` properties

This.args is supposed to be immutable, however this actually works.

```
@onChange={{set this "args.foo.bar"}}
```

Just FYI

The `set` helper is the entry point implicated by the report; start by tracing how it handles the `this.args.foo.bar` path. Confirm the intended behavior for immutable `this.args`, and consider the work done when the reported invocation can no longer mutate it.

Written by the indexing model from the issue text.