Investigate moving away from DO (Distributed Object) as IPC mechanism

MacVim currently uses Distributed Object as a way to communicate between individual Vim processes and MacVim but it's long been deprecated by Apple. See https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/DistrObjects/DistrObjects.html. As such, all usages of NSConnection are currently throwing deprecation warning.

Instead, Apple's preferred APIs are the XPC system: https://developer.apple.com/documentation/xpc

We should investigate whether to move off DO to either Apple's XPC or another IPC mechanism (for example, Neovim has its own IPC mechanism via msgpack). Since the DO implementation is the underlying system that we talks between MacVim GUI and Vim process, and there are a few quirks and back-and-forth messaging, we should make sure it keeps working (some edge cases includes window resizing with/without guioption-k, fullscreen, input, etc), and latency won't be affected as a result.

Also, occasionally we see error messages with DO (#1005), and I have seen issues with running unit tests locally if I have certain settings that seem to stem from DO init race connection, so hopefully we can eliminate those as well.

Start by locating the existing NSConnection and Distributed Object usage between the MacVim GUI and Vim process. Read Apple's XPC documentation and compare it with the alternative IPC approach mentioned in the issue. Done means identifying a viable replacement and validating communication, window resizing, fullscreen, input, latency, and the reported connection errors and test races.