Add Test Coverage badge to README.md

[warburec]

This may require moving of grammars into another sub-project or excluding from test coverage. If this is attempted, make sure it is easy to use the example grammars without requiring help from a build tool.

Badge creation examples:

• https://github.com/marketplace/actions/dynamic-badges
• https://dev.to/thejaredwilcurt/coverage-badge-with-github-actions-finally-59fa