[SE-0466] Under main actor default isolation, explicit nonisolated is not special

Given an explicitly-nonisolated type such as

    nonisolated struct S { }

all extensions of S were also being treated as nonisolated. This meant
that being implicitly nonisolated (i.e., when you're using nonisolated
default isolation) was different from explicitly-writing nonisolated,
which is unfortunate and confusing. Align the rules, such that an
extension of S will get default isolation:

    extension S {
      func f() { } // @MainActor if we're in main actor default isolation
    }

include/swift/Basic/Features.def

@@ -332,6 +332,10 @@ EXPERIMENTAL_FEATURE(LayoutPrespecialization, true)
 
 EXPERIMENTAL_FEATURE(AccessLevelOnImport, true)
 
+/// Disable the special behavior of of explicit 'nonisolated' vs. being
+/// implicitly nonisolated.
+EXPERIMENTAL_FEATURE(NoExplicitNonIsolated, true)
+
 /// Enables a module to allow non resilient access from other
 /// modules within a package if built from source.
 EXPERIMENTAL_FEATURE(AllowNonResilientAccessInPackage, false)