Fix definition and docs for setters

[varungandhi-src]

As flagged in #94 (comment), right now, we misinterpret a setter definition as a getter definition when emitting the definition and doc comment.

https://github.com/sourcegraph/lsif-typescript/blob/860ea23f777ab1a707cf69ed2c2242f157c393ea/snapshots/output/syntax/src/accessors.ts#L13-L15

We should fix that.