gh-127627: Add posix._emscripten_debugger function

[hoodmane]

• Issue: Emscripten: Add function to add JS breakpoint #127627

[hoodmane]

I guess since it's private it doesn't make it onto the os module.

[mdboom]

LGTM.

[freakboy3742]

👍 Seems like a straightforward addition. I guess it makes sense to keep it private initially; it's easy enough to promote to a public API if it seems worthwhile to do so in future.

[hoodmane]

I suspect this is very niche -- it's useful for debugging and fixing faulty emscripten syscalls. But perhaps other people will find more common uses for it.