Add coverage workflow to GitHub Actions

[Pennycook]

This action performs a coverage analysis for any source files that have been modified, and reports if there are any new or modified lines that are not covered by any tests.

The coverage analysis is focused on new code because we know there are paths that are currently not covered; the intent of this new action is only to prevent a pull request from making the situation worse.

Related issues

Closes #47. 🎉

Proposed changes

• Add a new GitHub action to check coverage.
• Add check-coverage.py to:
  • Read a coverage.json file;
  • Parse the git blame information for each missing line; and
  • Report any lines where blame points to a commit SHA that was passed to the script via its --commits argument.
• Add coverage.yml to:
  • Run coverage;
  • Generate coverage.json for only the files that were modified by the PR; and
  • Run check-coverage.py on coverage.json.