Add code coverage

What is the new feature

Add code coverage.

How is it implemented

Directly add in pipeline. The total coverage percentage will be shown in the MRs.
Reference: coverage, coverage visualization

• Some CMake/FortUTF files are picked up in the coverage. Find a way to remove them.
• Backtrace test fails in sphinx runner.
• Ragel generated files cannot be processed by gcovr.

• Backtrace function and codes generated by Ragel are not supported in this version.

Mandatory steps before review

• Gitlab CI passes (Hint: use make format for linting)
• New feature is covered by additional unit tests
• Mark the merge request as ready by removing Draft:

Mandatory steps before merge

• Reviewed by a maintainer
• Incorporate review suggestions
• Remember to edit the commit message and select the proper changelog category (feature/bugfix/other)

You are not supposed to merge this request by yourself, the maintainers of fortan-support take care of this action!