lake command to detect if manifest is out of date without running build or exe

[semorrison]

In any project use lake, there will need to be a CI step checking for warning: manifest out of date: messages from lake.

Currently as far as I can see you have to either run lake build or lake exe to see this warning. For testing purposes, there needs to be a way to check if this message will be generated by lake build or lake exe, but without committing to doing any work.

Hopefully this issue will be resolved just by someone telling me the relevant command, but I'll put this up as an issue as a reminder to install the CI step in Mathlib when this is resolved!

[Kha]

A build --dry-run might be nice to have in general