Add Coverage Diff on PRs to GitHub CI

[ye-spencer]

Goal:

• Create indicator of decreased coverage from main in PR (most likely a comment on the PR, not an outright CI failure)
• Create a comment showing changes in coverage from main; only for files that actually changed in coverage
• Have an indicator of coverage decrease
• There should be configurable parameters such as level-of-total-coverage-to-fail-under (default 0), fail-under-this-much-decrease (default 100)
• Add a command that can be ran in a specific environment to view per-line coverage

[ye-spencer]

One note is that the comments are NOT sticky; there will be one comment per run, which may get annoying. An initial look into how to do to sticky comments looked like a lot of work, but it can be a next step.

[ye-spencer]

Also, it seems the CI will not run on this PR because it is from a fork. See actions in: ye-spencer#9