Suggestion: add CI to block PRs

[alexcanessa]

Is your feature request related to a problem? Please describe.
There is no CI running on merging PRs at the moment.

Describe the solution you'd like
When a PR gets created, CI checks the code and prevents if there are broken tests or the coverage (both test and type) are below the threshold.

Describe alternatives you've considered
CircleCI connected with the lint, test and (obviously) this package to check that the coverage stays the same.

[tsaiDavid]

Wondering if you have ideas or experience in testing packages like this one, in which the unit tests might involve an actual mock repo or structure? I feel like that could be helpful to ensure we mirror the actual usage of this tool.

[alexcanessa]

I did mock the lint function from type-coverage-core but at a very basic level + done some tests.
I'd like to have both unit and integration testing - I think that's all you need in a repo like this.