What problem does this feature solve?

Basically to convert every image to webp (certain SEO use cases) or avif with cleaner and more practical code:
You can write:

conversion: [
  { from: '*', to: 'webp' },
  { from: 'JPG', to: 'jpeg' },
],

Instead of:

conversion: [
  { from: 'jpeg', to: 'webp' },
  { from: 'png', to: 'webp' },
  ... // All the other formats you use
  { from: 'JPG', to: 'jpeg' },
],

Not really integral, but nice to have, down to make the pr if approved.

What does the proposed API look like?

So, right now it matches the exact extension:

const userConversion = this.options.conversion.find(
  (item) => item.from === ext,
);

You can also afterwards check whether or not there is a *

const userConversion = this.options.conversion.find(
  (item) => item.from === ext,
) ?? this.options.conversion.find(
  (item) => item.from === '*',
);

Or, you could also make a matcher map on creation of the ImageProcessor object, but since there aren't realistically a large abundance of options, id say that an extra find call is fine for performance.