This converts the remark plugin to an Integration (which adds a configured the remark plugin itself to the remarkPlugins array).
Overall this makes the user experience better by adding a more familiar interface for configuration than the remarkPlugins + hacky remarkImages config the package used before.
The move to an integration also opens up the door for later stability+perf improvements regarding the build step, such as concurrency.
Changes
Configuration is now typed and done through the Integration rather than through the untyped + made up remarkImages config attribute (v1.0)
Moved from ['jpeg', 'webp'] as the default formats config to ['auto']. This resolves #7
Added an altRequired config for use cases where alt text can't be guaranteed. This resolves #8
Stability improvements, now that the config doesn't have to be wrangled through the filesystem and retyped