Increase Code Coverage to 90%

[avivkeller]

Ref: #304

Our code coverage is currently less than 45% 😬. That is not optimal, since this library has a lot of moving pieces, and many of which are untested.

I propose we:

• Enable code coverage checks (@nodejs/tsc needs to add Codecov's GitHub bot to this repository)
• Set patch coverage threshold to 80%
• Increase coverage to X%, set project coverage threshold to X% for every 10% until 90%.
  Currently: 70% Threshold
• Disable patch coverage threshold, rely on the project threshold

[anonrig]

@avivkeller It seems codecov is already enabled for this repository: https://app.codecov.io/gh/nodejs/api-docs-tooling

[avivkeller]

@avivkeller It seems codecov is already enabled for this repository: https://app.codecov.io/gh/nodejs/api-docs-tooling

Codecov is enabled, but the bot and status checks have not been added. https://github.com/apps/codecov

Sorry if that wasn't clear

[AugustinMauroy]

We could already exclude legacy test coverage because we don't care about it 😝

[anonrig]

@nodejs/tsc I've enabled codecov github app for this repository.