Add Vapor 3 API shims

[tanner0101]

Adds _Vapor3 module that contains deprecations for Vapor 3 APIs (#2373).

To use this module, add the product as a dependency to your App target in Package.swift. For example:

.target(name: "App", dependencies: [
    .product(name: "Vapor", package: "vapor"),
    .product(name: "_Vapor3", package: "vapor"),
]),

Then import the _Vapor3 module to enable deprecations.

import Vapor
import _Vapor3

...

app.get("test") { req -> Future<String> in
//                       ^~~~~~
// 'Future' is deprecated: renamed to 'EventLoopFuture'
    ...
}

Note: This module only deprecates part of the Vapor 3 API. Some things are missing and not everything can be deprecated. If you'd like to add something, please open up a PR!

[MrLotU]

Should we follow the NIO naming scheme here and name it _Vapor3APIShims?

[tanner0101]

These changes are now available in 4.14.0