Support retrieving specific repository files before executing script

Start by reading the action's existing input handling and script execution flow. Determine how a files input could retrieve selected repository files through the GitHub API before the main script runs, and define completion as those files being available for require() without a full checkout.

Is your feature request related to a problem? Please describe.

1. Writing javascript in YAML is less than ideal. Most editors won't fully support syntax highlighting, linting, etc.
2. Using the checkout action first, a user can require() a local script and run it, but performing a full checkout first for large repositories for workflows that only need to run a single script is less than optimal.

Describe the solution you'd like

1. Add an additional input such as files which can be retrieved via the github API if not present, before the main script is run.

Describe alternatives you've considered

1. Checking out the entire repository, then using require() and run.
2. Using other 3rd party actions to load one or more repository files, then require() and run.
3. Role our own to do the same thing
4. Requesting actions/checkout add a files input to reduce retrieved content and use that here.

Additional context
I find writing actions and workflows, even using act, to be a painful experience in general. Any additional features of an action that ease the pain somewhat are greatly appreciated. Using multiple actions to simulate something that an action could do on its own works, but it's less than ideal.