Support list input type

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Describe the enhancement
An action should be able to have an input type which is a list of strings. The toolkit should provide a getInput overload or new method to retrieve it.

Code Snippet

let vals: string[] = core.getListInput('inputName');

Additional information
Add any other context about the feature here.

Start at the core.getInput entry point named in the issue and trace how toolkit inputs are retrieved. Compare the requested getListInput('inputName') TypeScript usage with the current API; done means list-of-string input retrieval is supported and its behavior is covered by the relevant tests.