Support WebP format

WebP format would give us better quality with smaller filesize: https://developers.google.com/speed/webp

Puppeteer doesn't seem to support WebP out of the box (https://github.com/puppeteer/puppeteer/issues/5348), so it would require capturing first in png and then reformatting to WebP before saving.

Puppeteer API docs for reference https://github.com/puppeteer/puppeteer/blob/d615dd3d2c79f12fad81445dd14a4943141bda04/docs/api.md#pagescreenshotoptions

cc @sgomes @josephscott

Start with the Puppeteer Page.screenshot API documentation linked in the issue, then trace mShots's screenshot-saving path. Confirm how PNG capture and subsequent image conversion are handled, and define done as producing saved WebP thumbnails with the expected quality and smaller file size; the issue names no repository files or tests.

Written by the indexing model from the issue text.