[std] Add a better way to wrap shebang scripts

[kylewlacy]

Here's a list of places where we have custom code to wrap scripts:

• python
• moreutils
• node.npmInstallGlobal ("wraps" symlinks instead of shebang scripts, but very similar idea)

std.autopack() has an option to wrap shebang scripts, but very few packages are able to use it due to it not wrapping scripts in a suitable way. Most commonly, this is because std.autopack() references the interpreter binary directly, but the interpreter needs to be able to reference other files included by default (e.g. site packages that are included along with Python and Perl)