proposing additions for type annotation

[smoothdeveloper]

Proposing few examples on how to use whitespace for type annotations.

[dnfclas]

All CLA requirements met.

[cartermp]

Some minor changes, but otherwise this is great. Thanks @smoothdeveloper

[auduchinok]

I've seen a lot of code with no space before colon in both values and function definitions, like this:

// values:

let x: SomeType = y

let f () =
    let y: AnotherType = g x
    ()


// functions:

let f (arg: _): unit =
    ()

let f x: unit =
    ()

Perhaps, I've seen even more code formatted like this than with the space like in let a : int = 1.

Also, having different styles for patterns that are parameters and head patterns in bindings adds a lot of confusion on how these should be formatted, since seemingly same things get formatted differently.

@cartermp @dsyme I think we should explicitly allow this style, since Fantomas uses rules defined in this guide to enforce arguably less popular code style without any option to keep the no-space style. What do you think?

[dsyme]

Hmmm for me only the return type of functions should add whitespace before the :

let f (arg: _) : unit =
    ()

This is partly so ): doesn't look like a weird sad backward smilely.

For values there should be no whitespace:

let x: SomeType = y

[dsyme]

@auduchinok I made a proposed adjustment here: #24643