Add support for macOS signing.

[bruce-one]

What this PR does / why we need it:

This PR fixes macOS signing in the same way that pkg does.

Which issue(s) this PR fixes:

Fixes #446

Special notes for your reviewer:

I've tested these steps manually (using ldid to try the signing step), but haven't tested them as part of the nexe codebase as I don't have access to a mac... got some access :-) All working here :-)

The impl isn't using any streaming or any such, but the binary shouldn't be big enough to cause any real memory pressure I'd have thought? But perhaps it could be tweaked to work via streams or similar, for now I just wanted to create the draft PR in case it can help others :-) (And perhaps cleanup is a later thing? 🤷 :-) )

Until it's releasable, something akin to the following would work as a workaround:

build.js

const isDarwin = require('os').platform() === 'darwin'
nexe.compile({
  output: 'blah',
  patches: [(compiler, next) => isDarwin ? compiler.replaceInFileAsync('lib/internal/process/pre_execution.js', '16000)', '1600000)').then(next) : next()]
  ...
}).then(() => {
  if (isDarwin) {
    const { patchMachOExecutable } = require('pkg/lib-es5/mach-o')

    writeFileSync('blah', patchMachOExecutable(readFileSync('blah')))
  }
})

or, if done via the command line:

$ nexe --output output-blah --patch as-above-in-a-file.js ...
$ npm i pkg
$ node -e 'const target = process.argv[1]; fs.writeFileSync(target, require("pkg/lib-es5/mach-o").patchMachOExecutable(fs.readFileSync(target)))' output-blah

[bruce-one]

(Actually; this might break the nexe execution; just double checking :-( ) solved :-)

[calebboyd]

Thanks for opening this. Lets leave it as reference and address it after zipfs is in?