Support (and provide tests and examples for) multithreading

Now that WASI-SDK 34 has been released with `wasm32-wasip3` support for pthreads via the Component Model's cooperative multithreading feature, we should be able to support multithreading in Python code. Alex Crichton has [run the CPython test suite with multithreading enabled](https://github.com/WebAssembly/wasi-libc/pull/850) with a few patches. In theory, things should "just work" once we apply those patches to the CPython fork we're using in this project.

Note that #242 is a prerequisite for this.

Start by reviewing prerequisite issue #242, the CPython fork used by this project, and the patches discussed in wasi-libc PR #850. Enable the WASI-SDK 34 pthreads support, then verify the CPython test suite and add the requested multithreading examples and tests.

Written by the indexing model from the issue text.