🚚 Move to MQT Core package

[burgholzer]

Description

This PR replaces the QFR submodule with the newly introduced MQT Core library that subsumes the QFR, DD package, and ZX package. At the moment, this entails no functional changes and only required minor adaptions.

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 ✔️

No problems need attention.

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[codecov]

Codecov Report

Merging #145 (633e44c) into main (af7d70d) will not change coverage.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #145   +/-   ##
=====================================
  Coverage   85.6%   85.6%           
=====================================
  Files         20      20           
  Lines       1362    1362           
  Branches     190     190           
=====================================
  Hits        1166    1166           
  Misses       196     196           

Flag	Coverage Δ
cpp	79.5% <ø> (ø)
python	97.2% <100.0%> (ø)

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Impacted Files	Coverage Δ
mqt/qecc/cc_decoder/color_code.py	94.8% <100.0%> (ø)
mqt/qecc/cc_decoder/decoder.py	98.2% <100.0%> (ø)