PSA: Program launchers on UNIX are changing to shell scripts

[Sam Clegg]

I'm hoping to land the following change today: https://github.com/emscripten-core/emscripten/pull/10729

This changes the emscripten program launchers used on UNIX systems to be shell scripts: 

The program launchers are the filenames without any extension: emcc, em++, emar, etc.

This does not affect windows users. They use: emcc.bat, em++.bat, emar.bat, etc.

This does not affect the actual program code that lives `.py` files: emcc.py, emc++.py, emar.py.

The reason for this change is that there is no reasonable, cross platform, `#!` line that we can use to launch python.   For example on mac there is `python`, but not `python2`, and on the latest ubuntu release there is `python3` but not `python`.   This change allows us to run some shell logic to find a working version of python to run.

This also matches the current behaviour of windows which already uses .bat files (shell scripts for windows) to launch python.

The only users that might be affected by this change are those that run the extension-less files explicitly in python.  e.g.:  python emcc.  This will no longer work as you will be asking python to run a shell script.   The fix for such users is simply to run the `.py` file instead: python emcc.py.  If anyone out there uses this pattern I'd been keen to know and happy to help fix.

cheers,

sam