Move more extensions to new extension format

[JeremyKuhne]

Also add workaround in TypeExtensionTests for spurious CS8620 warning

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Note

Migrates extension methods to the new scoped extension(...) syntax across multiple areas, removes an unused span split helper, and adds null-forgiving operators in tests to silence CS8620 warnings.

• Extensions (migrated to scoped extension(...) syntax):
  • System/Globalization/InternalDateTimeExtensions.cs: Wrap GetDate, GetTime, GetTimePrecise under extension(ref readonly DateTime); make InternalDateTime private.
  • Touki/Io/StreamExtensions.cs (both Framework and library): Group Stream, StreamWriter, and TextWriter helpers into respective extension(...) blocks; keep read/write and formatted write APIs.
  • Touki/TypeExtensions.cs: Move IsAssignableTo into extension(Type?).
  • Touki/Buffers/SpanExtensions.cs: Move SliceAtNull and CompareOrdinalAsString into extension(...) blocks; remove SplitToEnumerable.
  • Touki/Buffers/SpanReaderExtensions.cs: Move TryReadPositiveInteger into extension(ref SpanReader<char>).
  • Touki/EnumExtensions.cs: Group flag helpers into extension<T>(T) and extension<T>(ref T) blocks (query and mutating operations).
  • Touki/Text/StringBuilderExtensions.cs: Group append/format helpers into extension(StringBuilder).
• Tests:
  • TestSupport/TypeExtensionTests.cs: Add null-forgiving (!) on typeof(...) args to work around CS8620 analyzer noise; no behavior changes.

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