Revive Session tests

[Techatrix]

With this PR i have removed the dependency on stdio and instead added a std.io.Writer interface in Server.zig.
This allows us to intercept outgoing responses by the Server and more easily perform tests without having to run zls as a child process.

Additional notable changes include:

• move main to main.zig and expose a small public interface in Server.zig
• the arena allocator is not passed as an argument to various functions in Server.zig but instead stored in the Server struct.
• logging can't be done globally (because of the decoupling from stdio) Globals must go! #536
• tests now use std.testing.allocator

I wanted to store the writer in the Server struct itself and make it generic but i am not aware of a method to achieve this without having to wrap the entirety of Server.zig deeper scope.

[SuperAuguste]

This is absolutely amazing; LGTM! Thank you so much! Logger changes are a little jarring but slightly less intuitive code > global state every time :P