Add type hints for the return type of function or member

[kerams]

Potentially also for lambdas?

Sidenote:

fsharp/vsintegration/src/FSharp.Editor/Hints/InlineTypeHints.fs

Lines 17 to 24 in 92b2b5d

	match symbol.GetReturnTypeLayout symbolUse.DisplayContext with
	| Some typeInfo ->
	let colon = TaggedText(TextTag.Text, ": ")
	colon :: (typeInfo |> Array.toList)
	// not sure when this can happen
	| None ->
	[]

The None case above is hit when the symbol is a member (currently it does not pass through isValidForHint). The empty hint parts then cause a crash later on with ToImmutableArray.