Use `main` field as default `path`

Instead of need to set the path where size-limit will start to process files, use the package.json file main field (if missing, it’s default value index.js) as default value to be used. I think it can be a good sane default and don’t require to config size-limit, or maybe also to install it globally and just do some checking in random modules without need to modify them to add the config entry…

[ai]

If you don’t have time, I will find another developer to do it, it is not a problem (your idea is good, that is the main step here)

[piranna]

Thank you! 😃