Allow for a custom search filtering function for the SelectionPrompt

[jord-nijhuis]

Is your feature request related to a problem? Please describe.
Right now, when using SelectionPrompt<T>.EnableSearch(), the search function will always be a String.Contains() see (ListPromptState.cs#L129).

It would be nice to allow for alternate searching strategies.

Describe the solution you'd like
A possible solution is to add an optional parameter to SelectionPrompt<T>.EnableSearch that is a Func<string, ListPromptItem<T>>, bool> that allows the user to provide a custom function that filters the items based on the search input.

If you want to search based on the start of the string, for example, you could do:

// ...
prompt.EnableSearch((search, item) => item.ToString()?.StartsWith(search, true, StringComparison.OrdinalIgnoreCase) == true)

Describe alternatives you've considered
An alternative option would be to add a separate method for setting the search function, instead of using SelectionPrompt<T>.EnableSearch.

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