[code_builder] control-flow refinements

[jason-mayer]

Adds some refinements for control-flow support (#2135). None of these changes are breaking as the control-flow APIs have not been published yet.

• Moved almost all logic to visitors, making the code much easier to follow. This also allowed for the removal of many internal-only intermediary classes.
• The body field on control-flow blocks is now type Code? instead of Block. This brings it into alignment with the body field on other specs.
• Reduced API leakage. Each control flow class now has its own visitor (with logic shared internally) rather than leaking ControlBlock via a common shared one. Branch and ControlExpression are now also exported, as they were being leaked.
• Renamed TryCatch -> Try and CatchBlock -> Catch to be more idiomatic and concise.
• Removed the ControFlow extension and moved all its methods to Expression proper to reduce scope noise.
• Added some quality of life functions to control flow classes such as from constructors and addCode/addExpression.
• Updated changelog, documentation, and tests to reflect these changes.

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Code Review

This pull request refactors the control-flow specifications in code_builder to simplify their structure and usage. Key changes include renaming TryCatch to Try, CatchBlock to Catch, and ControlFlow to ControlExpression, refactoring Conditional and Branch classes, and moving extension methods directly onto Expression. Additionally, loop and block bodies have been updated from Block to Code? to allow more flexible code generation. Feedback on these changes highlights a bug in Try that prevents valid try-finally blocks without catch clauses, a need for validation in Conditional to ensure only the last branch can omit a condition, and a request to expand documentation and provide usage examples for the newly public Branch, Catch, and Try APIs to align with the repository style guide.