Un-deprecate contiguous ASCII

[parkera]

The swift stdlib deprecated _isContiguousASCII, but Foundation depends on it for String extensions.

Is there a better entry point to use, or should we remove the deprecation from stdlib?

/swift-foundation/Sources/FoundationEssentials/String/String+Essentials.swift:109:70: warning: '_isContiguousASCII' is deprecated
            if encoding == .utf8 || (encoding == .ascii && str._guts._isContiguousASCII) {

[glessard]

The stdlib should at least have a failable String initializer that can validate ASCII. I think that could replace the current uses we make of the _isContiguousASCII property.

[jmschonfeld]

This was undeprecated with swiftlang/swift#70726 so I think we can go ahead and resolve this