test: consolidate and improve coverage reports

[acuarica]

Description:

This PR consolidates and improves coverage reports. More specifically

• it adds c8 package to provide V8 native coverage for the JS library,
• it uploads coverage reports from our test suites to Codecov in CI, and
• it adds a Coverage section to README to display coverage overview and instructions how to get coverage reports locally.

Related issue(s):

Fixes #280.

Notes for reviewer:

Checklist

• Documented (Code comments, README, etc.)
• Tested (unit, integration, etc.)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.74%. Comparing base (93da076) to head (369e0a6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #279      +/-   ##
==========================================
+ Coverage   81.25%   88.74%   +7.49%     
==========================================
  Files          12       20       +8     
  Lines        1083     2302    +1219     
  Branches      186      186              
==========================================
+ Hits          880     2043    +1163     
- Misses        148      205      +57     
+ Partials       55       54       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[andrewb1269]

Review and approve:
test.yml
.gitignore
README.md

[quiet-node]

LGTM