Asynchronous variant of AlternateScreen

[zensharp]

Is your feature request related to a problem? Please describe.
Currently, alternate screen buffers run synchronously which makes it difficult to use with async/await.

Describe the solution you'd like
A method named AlternateScreenAsync which behaves identically to AlternateScreen, but can be awaited.

Describe alternatives you've considered

• IExclusivityMode.RunAsync - It seems possible to achieve this through IExclusivityMode.RunAsync, but would require manually checking for capabilities and printing the correct control sequences (both of which are already handled by AlternativeScreen)

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