👷 MLIR - Set up coverage collection in CI

[burgholzer]

Problem Statement

We should be tracking the code coverage of the MLIR subcomponent of MQT Core just as we are tracking it for the regular code.

Proposed Solution

I'd expect this to need some reading on best practices for that in the MLIR ecosystem. This will also need adjustments to the codecov setup (https://github.com/munich-quantum-toolkit/core/blob/main/.github/codecov.yml), where a new "MLIR" flag probably makes sense.