implement nullable prompt

[danielklecha]

I implemented a solution to use TextPrompt with nullable types.
I created new interface INullablePrompt.
I was forced to change type of Validator in TextPrompt.
Example:

decimal? nullableDecimal = await new TextPrompt<decimal?>("Value?")
    .AllowEmpty()
    .ShowNullableAsync( _ansiConsole, CancellationToken.None );

@nils-a It's my proposition to fix #714.

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Please upvote 👍 this pull request if you are interested in it.

[dnfadmin]

All CLA requirements met.

[danielklecha]

@nils-a @patriksvensson Could you review my changes? Is it a desirable modification?

[danielklecha]

@microsoft-github-policy-service agree