Inspect own function properties

Replaces #70. Had to create a new PR because the old one had `v5` as the base, and apparently you can't change that after the fact.

We concluded offline that the diffs are fine, but that it would be ideal for the `inspect` output to include the extra properties. The best idea on the table was to put the extra properties in a comment, so I went ahead and implemented that.

As far as I can tell, the only thing that needs more work is the indentation of the extra properties when rendered as part of an object: https://github.com/sunesimonsen/unexpected/blob/e7843c24b833d5ad417c884495ccbc718a9f4790/test/unexpected.spec.js#L2743-L2745

@sunesimonsen Any idea for how to make the extra properties line up with `function`?