Support Ignite rendering from Vapor

[twostraws]

Apparently all we need to do is make the HTML protocol conform to ResponseEncodable. Presumably we can use #if canImport(Vapor) or something similar, so it's a conditional requirement?

[JPToroDev]

Since protocol extensions can't have inheritance clauses, in order to make this work, I think we'd need to make HTML a type alias like this:

#if canImport(Vapor)
typealias HTML = [NewHTMLName] & ResponseEncodable
#else
typealias HTML = [NewHTMLName]
#endif

[twostraws]

I was thinking something much more stupid 😅

#if !canImport(Vapor)
public protocol ResponseEncodable { }
#endif

That would mean HTML can conform to ResponseEncodable regardless of whether Vapor exists or not.

[JPToroDev]

Ah! I think that'd be fine. Does the same thing with less change, and an empty ResponseEncodable protocol doesn't do any harm.