Add subinterpreter support

[scopreon]

Issue number of the reported bug or feature request: #59

Describe your changes
This PR adds simple subinterpreter support to pystack. It preserves all original functionality but does it a few more times by traversing the interpreter linked list. It only supports pure python stacks currently.

Added a new component TracebackPrinter which should help with printing in future. Additional configuration for the printer can be added in future.

Few changes in version.h/cpp.

First contribution so maybe missed something.

Testing performed
Added tests for TracebackPrinter. Added additional end-to-end test for subinterpreters support using the new concurrent 3.14 functionality.

Additional context
Example output

root@875d1afe36dd:/workspaces/pystack# pystack remote 86996
Interpreter-3
  Traceback for thread 87005 (Thread-3 (exec)) [] (most recent call last):
      (Python) File "<script>", line 5, in <module>
  
Interpreter-2
  Traceback for thread 87004 (Thread-2 (exec)) [] (most recent call last):
      (Python) File "<script>", line 5, in <module>
  
Interpreter-1
  Traceback for thread 87003 (Thread-1 (exec)) [] (most recent call last):
      (Python) File "<script>", line 5, in <module>
  
Interpreter-0 (main)
  Traceback for thread 87005 (Thread-3 (exec)) [] (most recent call last):
      (Python) File "/usr/lib/python3.14/threading.py", line 1044, in _bootstrap
          self._bootstrap_inner()
      (Python) File "/usr/lib/python3.14/threading.py", line 1082, in _bootstrap_inner
          self._context.run(self.run)
      (Python) File "/usr/lib/python3.14/threading.py", line 1024, in run
          self._target(*self._args, **self._kwargs)
      (Python) File "/usr/lib/python3.14/concurrent/interpreters/__init__.py", line 215, in exec
          excinfo = _interpreters.exec(self._id, code, restrict=True)
  
  Traceback for thread 87004 (Thread-2 (exec)) [] (most recent call last):
      (Python) File "/usr/lib/python3.14/threading.py", line 1044, in _bootstrap
          self._bootstrap_inner()
      (Python) File "/usr/lib/python3.14/threading.py", line 1082, in _bootstrap_inner
          self._context.run(self.run)
      (Python) File "/usr/lib/python3.14/threading.py", line 1024, in run
          self._target(*self._args, **self._kwargs)
      (Python) File "/usr/lib/python3.14/concurrent/interpreters/__init__.py", line 215, in exec
          excinfo = _interpreters.exec(self._id, code, restrict=True)
  
  Traceback for thread 87003 (Thread-1 (exec)) [] (most recent call last):
      (Python) File "/usr/lib/python3.14/threading.py", line 1044, in _bootstrap
          self._bootstrap_inner()
      (Python) File "/usr/lib/python3.14/threading.py", line 1082, in _bootstrap_inner
          self._context.run(self.run)
      (Python) File "/usr/lib/python3.14/threading.py", line 1024, in run
          self._target(*self._args, **self._kwargs)
      (Python) File "/usr/lib/python3.14/concurrent/interpreters/__init__.py", line 215, in exec
          excinfo = _interpreters.exec(self._id, code, restrict=True)
  
  Traceback for thread 86996 (python3.14) [] (most recent call last):
      (Python) File "/workspaces/pystack/test_file_multiple_interps.py", line 28, in <module>
          print("Main interpreter sleeping forever...")

[codecov-commenter]

Codecov Report

❌ Patch coverage is 78.82883% with 47 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.13%. Comparing base (58dcac6) to head (6f9df58).

Files with missing lines	Patch %	Lines
src/pystack/_pystack/pythread.cpp	32.55%	29 Missing ⚠️
tests/integration/test_subinterpreters.py	48.38%	16 Missing ⚠️
src/pystack/_pystack/interpreter.cpp	88.88%	1 Missing ⚠️
src/pystack/_pystack/process.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #279      +/-   ##
==========================================
+ Coverage   83.03%   83.13%   +0.09%     
==========================================
  Files          48       50       +2     
  Lines        6242     6396     +154     
  Branches      480      489       +9     
==========================================
+ Hits         5183     5317     +134     
- Misses       1059     1079      +20     

Flag	Coverage Δ
cpp	83.13% <78.82%> (+0.09%)	⬆️
python_and_cython	83.13% <78.82%> (+0.09%)	⬆️

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