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

[cmeeren]

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

MyImpl(arg) :> MyInterface |> MyUnionCase

as

MyImpl(arg) :> MyInterface
|> MyUnionCase

whereas I'd suggest

MyImpl(arg)
:> MyInterface
|> MyUnionCase