[style-guide] Treat cast operators :> and :?> as pipe operators

I suggest that the style guide is updated to specify that the cast operators `:>` and `:?>` can be treated as pipelines when formatting.

For example, for a suitable max width, Fantomas currently formats

```f#
MyImpl(arg) :> MyInterface |> MyUnionCase
```

as

```f#
MyImpl(arg) :> MyInterface
|> MyUnionCase
```

whereas I'd suggest

```f#
MyImpl(arg)
:> MyInterface
|> MyUnionCase
```