Support array list of extensions for custom data file formats

[Snapstromegon]

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

Providing multiple file extensions like yml and yaml as a comma separated list is often fine, but especially when generating these extension mappings programmatically a list would be easier to use.
This is related to #2381.

Describe the solution you'd like

I would want this to work too:

eleventyConfig.addDataExtension(["yml", "yaml"], (contents, filePath) => {
  // …
});

Describe alternatives you've considered

No response

Additional context

Like always I'd be willing to contribute a PR resolving this.

[zachleat]

Code for this lives here: https://github.com/11ty/eleventy/blob/a9ee377494a68934a8233559e3986839bdff5d5e/src/UserConfig.js#L956

Tests can be added here: https://github.com/11ty/eleventy/blob/a9ee377494a68934a8233559e3986839bdff5d5e/test/UserDataExtensionsTest.js