Evaluate usage of `tsd` to control breaking changes in type Definitions

[oscard0m]

We should start by creating a failing test using SamVerschueren/tsd to verify that we actually fixed the problem and to make sure that we won't break it again.

Is this still necessary? Do I create an issue for this? @gr2m

Originally posted by @dominguezcelada in https://github.com/octokit/webhooks.js/issue_comments/705478219

[gr2m]

this PR introduces tsd to Probot: https://github.com/probot/probot/pull/1389/files. I think it looks pretty great!