Add ControlBuilder.Memory

[scottbilas]

Span<> isn't available inside of an async method, which makes it more difficult to use ControlBuilder without an alloc.

The underlying type is ArrayBufferWriter<>, which exports a Memory, so this change just exposes that API to ControlBuilder as well.

[alexrp]

I don't have any particular problem with taking this change, but just out of curiosity, what's the situation that made the Memory property necessary?

[scottbilas]

The use case is when wanting to pass ControlBuilder memory to an async utility function. The compiler will not permit receiving it as a ReadOnlySpan<char>. So ControlBuilder.Span is unusable.

A good example of this is Terminal.OutAsync(ReadOnlyMemory<char>). I can't get at the underlying ArrayBufferWriter array memory in the ControlBuilder, so the only other option is an unfortunate allocating ToString().