Add CodeCov reporting back after a 3 year hiatus

[filmaj]

This should bring codecov reporting back in for node-slack-sdk; woot!

• Removed nyc as the coverage tool, replaced with c8, it hasn't been updated in 4 years and c8 is essentially a drop-in replacement and maintained. Hat tip to @zimeg as I just followed their lead in the packages/cli-hooks sub-package.
• Modified GItHub Actions CI to upload code coverage, per sub-project, using CodeCov Flags, when running on latest node, and if the sub-project creates a coverage report. Some projects don't have that level of testing, so first checking for the existence of a coverage report before firing off the CodeCov GH Action.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.58%. Comparing base (84450fd) to head (f96b50a). Report is 542 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1788 +/- ## ========================================== - Coverage 94.41% 88.58% -5.84% ========================================== Files 12 18 +6 Lines 806 6148 +5342 Branches 178 297 +119 ========================================== + Hits 761 5446 +4685 - Misses 15 690 +675 + Partials 30 12 -18 ``` | [Flag](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/1788/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | Coverage Δ | | |---|---|---| | [cli-hooks](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/1788/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `95.07% <ø> (?)` | | | [eventsapi](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/1788/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `?` | | | [interactivemessages](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/1788/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `?` | | | [oauth](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/1788/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `76.51% <ø> (?)` | | | [socket-mode](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/1788/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `59.41% <ø> (?)` | | | [web-api](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/1788/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `96.48% <ø> (?)` | | | [webapi](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/1788/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `?` | | | [webhook](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/1788/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `95.20% <ø> (+7.70%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi#carryforward-flags-in-the-pull-request-comment) to find out more.