Support list input type

[bryanmacfarlane]

Thank you 🙇‍♀ for wanting to create an issue in this repository. Before you do, please ensure you are filing the issue in the right place. Issues should only be opened on if the issue relates to code in this repository.

• If you have found a security issue please submit it here
• If you have questions about writing workflows or action files, then please visit the GitHub Community Forum's Actions Board
• If you are having an issue or question about GitHub Actions then please contact customer support

If your issue is relevant to this repository, please include the information below:

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.

[kiprasmel]

I was going through https://github.com/actions/cache trying to implement multiple path support, and when going through source code, I only noticed the core.getInput function, but not one for core.getInputs to get multiple of them.

Then, later, I found this:

const restoreKeys = core
	.getInput(Inputs.RestoreKeys)
	.split("\n")
	.filter(x => x !== "");

so, is

let vals: string[] = core.getInput('inputName').split("\n").filter(x => x !== "");

the official way of doing it?

If so, I've created a PR to implement this: #336