✨ Allow project installation

[burgholzer]

Description

This PR sets up the project so that it can be installed properly.
As it is configured right now, this allows to install the mqt-core Python package and make the C++ project available via find_package(mqt-core) as long as the proper cmake directory is added to the search path.
The right directory can easily be determined via python -m mqt.core --cmake_dir (or via mqt-core-cli --cmake_dir).
You might be wondering what this is useful for. It allows us to only require a single source copy of mqt-core in top-level projects once we switch them to using the mqt-core Python package. Otherwise, we would need to maintain the submodule and the Python package in parallel.

While I expect this to require some follow ups once used in practice, I believe this is a solid start.

Fixes #504

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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f450d97) 91.3% compared to head (7ff1c87) 91.3%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #529   +/-   ##
=====================================
  Coverage   91.3%   91.3%           
=====================================
  Files        128     130    +2     
  Lines      13805   13851   +46     
  Branches    2151    2151           
=====================================
+ Hits       12609   12655   +46     
  Misses      1196    1196           

Flag	Coverage Δ
cpp	91.0% <ø> (ø)
python	99.7% <100.0%> (+<0.1%)	⬆️

Files	Coverage Δ
src/mqt/core/__init__.py	100.0% <100.0%> (ø)
src/mqt/core/__main__.py	100.0% <100.0%> (ø)
src/mqt/core/commands.py	100.0% <100.0%> (ø)