SE-0478: Default actor isolation typealias

[ksluder (Kyle Sluder)]

@allevato

• The default types of inferred literals can be changed by setting typealiases like typealias IntegerLiteralType = UInt8 in a file. Likewise for strings, floating point values, and a few others.
• The distributed runtime allows the default actor system to be changed by declaring typealias DefaultDistributedActorSystem = Something.

The proposed ability to toggle “warnings as errors” in a specific file does not fit into this mold.

However, these examples do fit into a more general pragma-like feature (which is spelled #compilerSettings in the linked pitch). One can imagine #compilerSettings(defaultDistributedActorSystem: "Something") being a supported syntax that can be copy-pasted between a source file and a SwiftPM manifest.