add `format: 'webp'` to support Astro automatically.

[hexiro]

Description

Hello, first I want to say thank you for making this amazing package! this is exactly what I was looking for! I have absolutely no complaints but wanted to suggest something to see if you would be interested in adding this to your package.

I am using Astro with this plugin and Astro's image component requires the props src, width, height, and format. using your plugin with ?lqip, lqip is of course added but format is left behind. this is far from the end of the world as the solution i have been using (pictured above) works lovely.

with that being said, I don't think it would neccesarily be a bad idea to pass to output the format even though it's just going to be webp everytime, so that it automatically satisfies Astro's ImageMetadata type, and becomes even more out-the-box for those users.

Please let me know what thoughts you have on this! and once again this package is easy to use and convenient! thank you so much!

• [X] I’m willing to open a PR (see CONTRIBUTING.md)