ci: Report coverage with CodeCov

[exoego]

Closes #2857
Closes #1391
Closes #175

This PR configures Vitest to generate coverage in JSON format per each test suite.
Multiple coverage files are uploaded into CodeCov, then CodeCov automatically merges those uploaded coverages smartly.

exoego#3 is a working demo how CodeCov bot reports coverage changes on each PR
You can also see the demo coverage on my CodeCov account:

(this demo coverage is not "accurate" since it includes unexpected files like benchmarks and runtime_tests. Such unexpected files are excluded in this PR)

⚠️ Attention ⚠️

Repo administrator should set CODECOV_TOKEN as GitHub secret for Actions.
The token can be obtained in https://app.codecov.io/github/honojs/hono/settings
or maybe https://app.codecov.io/github/honojs/hono

Note

CodeCov is free for open-source repos while some enterprise-level features are not available.
https://about.codecov.io/pricing/

This PR does not enable coverage report on terminal (text reporter).
Because it is required to merge multiple coverage.
I will implement that in a separate PR if needed.

You can configure behavior in codecov.yml. Refer https://docs.codecov.com/docs/common-recipe-list#set-non-blocking-status-checks

The author should do the following, if applicable

• Add tests
• Run tests
• bun run format:fix && bun run lint:fix to format the code
• Add TSDoc/JSDoc to document the code

[codecov-commenter]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered ☂️

[yusukebe]

@exoego

Awesome! I've never used CodeCov, but this seems so great! Let's go with it. Merge now, and then I'll set the CODECOV_TOKEN. Thank you!