Implement Multi-Phase Init for _xxsubinterpreters

[ericsnowcurrently]

See PEP 630 and PEP 687. This is an internal test module so the bar isn't as high as for regular stdlib modules.

Linked PRs

• gh-99741: Implement Multi-Phase Init for the _xxsubinterpreters Module #99742
• gh-99741: Fix the Cross-Interpreter Data API #99939
• gh-99741: Clean Up the _xxsubinterpreters Module #99940
• gh-99984: Fix Compiler Warnings #100036