Accept standard argument formats for "false" attributes passed through query args

_(Leaving this issue here just so I don't forget about it over the weekend.)_

While troubleshooting an issue trying to bypass the automatic webp formatting with @smccafferty, we noted that none of the expected formats (`?webp=false`, `?webp=0`, `?webp=no`) for setting the `webp` query argument to false worked as we'd expected. I guess this makes sense, because URL query parameters are interpreted as strings both by the `URLSearchParameters()` js API and by AWS Lambda's `queryStringParams`. But it was surprising that 'false', '0', and 'no' all evaluated as true.

Sean was able to work out that setting the attribute to an empty string, like `?webp=`, works for disabling webp. But I'd like to see a more intuitive interface.

Start by locating where the webp query argument is read and converted into an attribute value in the TypeScript AWS service. Check how query-string values are currently interpreted, then define completion as intuitive false formats such as false, 0, and no disabling WebP consistently, with coverage for the supported formats.

Written by the indexing model from the issue text.