synchronize `Default (OSX).sublime-keymap` files with other platforms

[timotheecour]

ref: https://github.com/Varriount/NimLime/pull/133#issuecomment-445425846

maybe we could auto-generate these from a nim template? given that it's a package for Nim anyways, users should be expected to know nim so could edit the template removing code duplication would be nice