Restructure workspace: Create separate crates for client, server and utilities

[kunalmohan]

• Separates zellij into 4 crates- zellij, zellij-utils, zellij-client, zellij-server.
• zellij-utils contains all utilities shared by the client and the server. And zellij parses the command-line arguments and calls appropriate API methods exposed by zellij-client and zellij-server.
• Since cfg(test) is not propagated to the dependencies, zellij-client, zellij-server and zellij-utils now have a feature test which zellij enables while testing. So cfg(test) has been replaced by cfg(any(feature="test", test)) in the 3 crates.
• Makefile.toml has been modified to skip various workspace crates so that crates are compiled parallelly as a dependency, instead of compiling each crate explicitly in a sequential manner.

[khs26]

Does this change how you run zellij at all? It looks like it probably doesn't, but if it does, we should update the docs

[kunalmohan]

Does this change how you run zellij at all?

Nope, the commands are the same. I've made a few changes so that the build is not sequential, but the user interface is same. I haven't tested the install and publish commands, but I think they should work fine. I'm hoping that @TheLostLambda will be able to point out any potential bugs from a quick review of the Makefile.toml changes.