⏪ add MQT::CorePython target back

[burgholzer]

Description

As the integration of the mqt-core Python package into the top-level tools turned out to be a much lengthier process than imagined, this PR adds back the MQT::CorePython target that was previously used to read in Qiskit objects.
This should allow development in the top-level libraries to continue without waiting for the remaining issues with the Python package being resolved.

Checklist:

• The pull request only contains commits that are related to it.
• I have added appropriate tests and documentation.
• I have made sure that all CI jobs on GitHub pass.
• The pull request introduces no new warnings and follows the project's style guidelines.

[github-actions]

Cpp-Linter Report ⚠️

Some files did not pass the configured checks!

clang-tidy reports: 1 concern(s)

• include/mqt-core/python/qiskit/QuantumCircuit.hpp

  include/mqt-core/python/qiskit/QuantumCircuit.hpp:3:10: error: [clang-diagnostic-error]

  'pybind11/pybind11.h' file not found

  #include "pybind11/pybind11.h"
           ^~~~~~~~~~~~~~~~~~~~~

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