suggestion: use code coverage tool

[iuioiua]

Having a test coverage badge instils confidence in developers using this module. It also highlights where test coverage can be improved, possibly attractive to contributors. It'd be great if this module had one. Codecov appears to work great. For configuration examples, see:

• https://github.com/denoland/deno_kv_oauth/blob/main/.github/workflows/ci.yml
• https://github.com/denoland/deno_kv_oauth/blob/main/.github/codecov.yml