Added Code Coverage CI

[okBrian]

Description

• Added Code Coverage CI with the CodeCov Action.
• Added --gcov as a build compile option to build with coverage files.
• Added CodeCov Badge to README.md.
• Made the syntax for some of the other CI a bit cleaner.

Fixes #332

Type of change

• New feature (non-breaking change which adds functionality)

Scope

• This PR comprises a set of related changes with a common goal

How Has This Been Tested?

• Test that ./mfc.sh build -j 8 --gcov produced the coverage files without issue.
  • The coverage files are found in build/staging/[id of a staging]/CMakeFiles/[staging].dir/fypp/
• Test that the coverage CI works as intended and uploads the result on codecov

Test Configuration:

Ubuntu 22.04, Docker, and Github Actions
gcc-11, gcc-12, gcc-13

Checklist

• I have added comments for the new code
• I have made corresponding changes to the documentation (docs/)
• I ran ./mfc.sh format before committing my code
• This PR does not introduce any repeated code (it follows the DRY principle)
• I cannot think of a way to condense this code and reduce any introduced additional line count

[codecov]

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 ☂️

[sbryngelson]

Looks good! @henryleberre can review this PR since this is mostly code from him.

[henryleberre]

I fixed a merge conflict and made some small tweaks. @sbryngelson you can squash merge when you're ready.

[sbryngelson]

Will merge when CI passes