Support the PyEmscripten platform - #1083

Resolves #1089

Notes

• The errors that were resolved by the extension changes can be seen in this run.
• Some tests for this platform are skipped to avoid crashes such as the one in this run. For context, see:
  • Fatal RangeError: Maximum call stack size exceeded when combining partial with infinite recursion pyodide/pyodide#5959
  • Stack Management Issue(s?) pyodide/pyodide#5987
• I also refactored the matrix to allow for defining the target platform rather than just architecture. After this is merged, feat: add riscv64 to Linux wheel build matrix #987 should be implemented as a new matrix member rather than an entirely separate job.

References

• https://pyodide.org/en/stable/development/abi.html
• https://cibuildwheel.pypa.io/en/stable/platforms/#pyodide
• https://pyodide.org/en/stable/usage/packages-in-pyodide.html
• https://peps.python.org/pep-0783/

AI use

I collaborated with GPT-5.5 on Extra High for the following:

• The initial scaffolding for the CI changes (which are mostly no longer present because I disliked the separate job approach).
• The extension changes, primarily for understanding the root cause in order to simplify the logic and create informative code comments.
• A loop to determine if lowering the recursion limit would remove the need to skip some tests but it didn't help for any of them.