Generate ExpressibleAsProtocols using SwiftSyntaxBuilderGeneration

- Stub out ExpressibleAsProtocolsFile
- Declare protocols
- Add conformance extensions
- Remove `ExpressibleAsProtocols.swift.gyb`

Author: fwcd

Package.swift

@@ -77,7 +77,6 @@ let package = Package(
       dependencies: ["SwiftSyntax"],
       exclude: [
         "gyb_helpers",
-        "ExpressibleAsProtocols.swift.gyb",
         "BuildableBaseProtocols.swift.gyb",
         "BuildableCollectionNodes.swift.gyb",
         "BuildableNodes.swift.gyb",