Add an initializer to construct Syntax nodes from syntax protocol types

[ahoppen]

Resolving https://forums.swift.org/t/swiftsyntax-as-conversions/37166, we are missing an initialiser to create Syntax nodes from SyntaxProtocol (and ExprSyntaxProtocol etc.) types. This adds the necessary initialiser.

Note that I needed to add an argument label fromProtocol to the initialiser to avoid ambiguity with the following existing initialiser. I think it also makes it clear that we are dealing with existentials here, similar to how we have asProtocol etc.

public init<S: SyntaxProtocol>(_ syntax: S) {
  self = syntax._syntaxNode
}

[ahoppen]

@swift-ci Please test