Circuit visualization

We'd like to add the ability to generate circuit diagrams from a Q# program.

These can appear in:
- VS Code
- Python notebooks
- Python command line (ascii art)

Small programs without classical constructs (e.g. the Bell State sample) are fairly easy to generate diagrams for, but we'll have to figure how best to visualize more complex cases.

Design considerations:
- How to represent classical constructs, such as loops, visually
- Whether the circuit should map to the hardware behavior (i.e. closer to the generated QIR, showing auxiliary qubits, unrolled loops etc), or to the high level input Q# program
- UI to visually map parts of the circuit to source code

For reference, Q# notebooks in the classic QDK had the ability to generate diagrams using the `%trace` magic and the `quantum_viz` Python package.

No implementation files, tests, or entry points are named. Start by reviewing the classic QDK `%trace` reference and the `quantum_viz` Python package, then clarify the supported Q# constructs and target surfaces; done would require an agreed design and circuit diagrams for VS Code, Python notebooks, and the Python CLI.

Written by the indexing model from the issue text.