Move exec and shell_init from chef-cli to chef.exe

[mwrock]

On Windows, simply invoking chef-cli consumes ~3 seconds for it to do all of its gem loading and requiring before it ever gets to executing the actual command. That's just the joy of ruby on windows but these two commands should be straightforward to port to go (especially chef exec) and give windows users the gift of 3 seconds.

From what I can tell, the trickier part of implementing this in go is emiting the GEM_ variables. Emitting those from ruby in chef-cli, we can simply grab them from the current ruby environment. However I really wonder if we need these variables set. As long as the user's PATH is correctly configured, the bin stubs of the ruby tools in embedded/bin should force the ruby in that same directory. However I feel like we must be adding those for a good reason. At the least we could just shell out with something like ruby.exe -e "puts Gem.default_dir" which would still be much faster since it does not require any gem loading.

Aha! Link: https://chef.aha.io/features/SH-372