Scale feature

- Add an option "Resize and convert to webp"
- Allow to specify the size of the file.
- The extension will then add the option -resize to the command line

By default, we can preview image files in vscode. Another feature would be to add a gui button to crop and resize the image to webp.

Start by locating the extension's existing image-conversion command handling and how VS Code options are exposed. Define the scope between resize-and-convert support and the proposed crop/resize GUI, then verify that the requested size is passed through as a resize option and that the resulting image is WebP.

Written by the indexing model from the issue text.