add new nushell syntax for input output types

[fdncred]

JT added some new syntax to custom commands to allow one to specify the input and output types. You can see the details in PR9690

For example (a single int-to-int input/output pair):

def foo []: int -> int { ... }

You can also have multiple input/output pairs:

def bar []: [int -> string, string -> list<string>] { ... }

@glcraft I hope it's not a problem tagging you on new syntax. You're help would be appreciated here too.

Related to #146

[glcraft]

Yep ok, I'm going to check this out 👍

[fdncred]

closed by #160