feature: add barrier instruction

[nilsquet]

Issue #, if available:
#974

Description of changes:
Starting with the changes from #993 and #1002, the barrier instruction is implemented as a CompilerDirective. To this end, changes were necessary in a few files.

Testing done:
tox

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (b98a75e) to head (8db5a31).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1118   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          162       162           
  Lines        10387     10474   +87     
  Branches      1262      1276   +14     
=========================================
+ Hits         10387     10474   +87     

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[laurencap]

because self.qubits can be None?

[nilsquet]

The motivation was to not have any effect when adding a barrier to an empty circuit.

[nilsquet]

Since the linter was not happy anymore with the added complexity introduced here, I had to refactor the ascii_circuit_diagram.py and text_circuit_diagram_utils.py a bit.