Multigoal linter

Stream: PhysLean

[Joseph Tooby-Smith]

This is related to one of @Daniel Morrison reviews of my PR, where a good point was made concerning multi-goals and dot notation.

Should we try and turn the Multigoal linter on since it is not currently for PhysLean? This is related to this (almost year old) GitHub issue PhysLean#353. I think this will be a lot of work to fix all of the warnings it will give already, but maybe there is a way that we can turn it on only for new changes and then slowly go back and change it throughout the library. Or maybe this is somewhere AI can help.

[Daniel Morrison]

Personally I would be happy with that, and I would be open to making the upgrade