Add transforms to remove unused imports, variables, definitions and unreachable code

[SahilRajputdotexe]

This adds four opt-in transforms that remove dead code:

• remove_unused_imports (--remove-unused-imports): drops imported names that are never used
• remove_unused_variables (--remove-unused-variables): drops x = value when x is never used and the value has no side effects
• remove_unused_definitions (--remove-unused-definitions): drops functions and classes that are never used
• remove_unreachable (--remove-unreachable): drops statements after a return, raise, break or continue

They are off by default, because they can break code that is imported by something else or that relies on import side effects. Modules that use exec, eval, locals, globals, vars or a star import are left alone, and so are names in a literal __all__, dunder names, decorated definitions and class bodies.

Removing code can leave more unused, so the transforms repeat until nothing changes. For example, removing an unused helper() also removes the import json that only it used.

Unreachable code is only removed if that doesn't change how names resolve. A return followed by a yield stays, and so does a trailing assignment to a name that is read earlier in the function. A with body that only ends by raising doesn't count, because the context manager can swallow the exception.

There is a docs page for each option, a changelog entry and the stub arguments. The new tests pass on 3.8, 3.9, 3.11, 3.12 and 3.14 locally along with the existing suite, and I ran all four options over the standard library to check the output still compiles. The 2.7 and 3.3+ jobs run on CI.