Make FormatWithQuotesIfString public

Sometimes we may want to have more control when doing "UnParsing", for example I'd want to do the reverse of `UnParserExtensions.SplitArgs` (get a `string` from a `IEnumerable`).
By exposing `FormatWithQuotesIfString`, it would be much easier for me to implement a reverse version of `SplitArgs`.
https://github.com/commandlineparser/commandline/blob/d443a51aeb3a418425e970542b3b96e9da5f62e2/src/CommandLine/UnParserExtensions.cs#L230

(Of course, if it's possible to provide a `string UnParserExtensions.UnifyArgs(this IEnumerable args);`, it'd be even better!)

Open src/CommandLine/UnParserExtensions.cs at the referenced FormatWithQuotesIfString method. Confirm how it is currently declared and what public API conventions apply; done means callers can access this method to format string arguments for a reverse SplitArgs implementation.

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