Allow SelectionPrompt to have an initial selection

[reduckted]

fixes #508

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• I have commented on the issue above and discussed the intended changes
• A maintainer has signed off on the changes and the issue was assigned to me
• All newly added code is adequately covered by tests
• All existing tests are still running without errors
• The documentation was modified to reflect the changes OR no documentation changes are required.

Changes

SelectionPrompt now has a DefaultValue property that can be used to set the item that should be initially selected.

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[timothyparez]

This, for me, is the number one missing feature.
Often we presents the users with a large list of operations
until this PR they had to scroll through the list every time (because they often repeat commands).
Now we simply set the last command they executed as the default.

Just tested this and it works great:

var fruit = AnsiConsole.Prompt(
    new SelectionPrompt<string>()
        .Title("What's your [green]favorite fruit[/]?")
        .PageSize(10)```
+       .DefaultValue("Banana") // <--- Setting the default value
        .MoreChoicesText("[grey](Move up and down to reveal more fruits)[/]")
        .AddChoices(new[] {
            "Apple", "Apricot", "Avocado", 
            "Banana", "Blackcurrant", "Blueberry",
            "Cherry", "Cloudberry", "Cocunut",
        }));

AnsiConsole.WriteLine($"I agree. {fruit} is tasty!");