Make fsevents install a 'shim' when installed on non-OSX platforms

[lxe]

This will allow for removing fsevents as optionalDependency and will fix shrink-wrapping of projects that target multiple platforms.

ref: paulmillr/chokidar#86
ref: madec-project/ezvis#85
ref: npm/npm#6801
ref: npm/npm#2679

[bnoordhuis]

I don't think projects will be able to move away from having it as an optionalDependency.

npm will automatically try to run node-gyp (or node-pre-gyp) when it sees a binding.gyp in the top-level directory and that's going to fail when the user doesn't have a compiler installed (e.g. most Windows users.)

[es128]

I agree with @bnoordhuis. This would likely create a lot more pain than it would solve. Show us in a PR if you think we're wrong about that.

The issues with shrinkwrap and other various issues with the handling of optional and OS-specific deps are really npm's problem to solve.

Going to close the issue, but feel free to keep discussing.