ts-morph-structures: Implement statement structures missing from ts-morph

I started ts-morph-structures partly in response to an issue asking for [expression structures](https://github.com/dsherret/ts-morph/issues/683). Ultimately the issue was closed as not planned, and I had implemented _type_ structures.

Fast forward to today, and I actually do need statement structures. I'm dealing with a mix of structures and strings which represent class method statements. (`try { return super.ownKeys(...); } finally { /* do something */ }`).

I'll also need to implement my ["stage four / five" bootstrapping plan](https://github.com/ajvincent/ts-morph-structures/issues/82).

(sigh) I was hoping to avoid this.