Emscripten: Make python.sh function as proper Python CLI

[hoodmane]

There are several problems with python.sh right now:

1. It doesn't work correctly on macs with bsd coreutils
2. It doesn't mount the native file system into the Emscripten file system, so python.sh /some/file.py will fail to find the file
3. It doesn't set the current directory in the Emscripten file system to the same as the host working directory so python.sh file.py won't look for file.py in the current directory
4. It doesn't inherit environment variables

@freakboy3742

Linked PRs

• gh-127503: Emscripten make Python.sh function as proper Python CLI #127506
• gh-127503: Fix realpath handling in emscripten cli #127632
• gh-127503 Don't propagate native PATH to Emscripten Python #127633
• gh-127503: Improve tracebacks on Emscripten when there is a trap #131158