Implement linter options

[odersky]

Scala 3 is currently missing several of Scala 2's linter options, including:

• unused variables
• unused imports

It would be great to add these.

We could also look out for other valuable options to add.

[odersky]

If this project is taken as a semester project, we would have to synchronize with the state when the project starts. reserving the project as a semester project should not hold up progress that could be made before.

[griggt]

Note that there is already a PR for unused imports: #14524

[odersky]

Yes, @som-snytt what's the status on that? I notice it is still in draft state?

[griggt]

I can probably make some time to help with that one if needed.

[som-snytt]

I'll rebase -Wunused:imports for further @griggt feedback. I don't think -rewrite option is a blocker, but it's important to handle language imports in a way that is friendly to cross-compilation. Thanks again, griggt. (Also I don't want to add to anyone's daily cognitive load if a feature isn't a priority.)

[odersky]

Project reserved for Paul Cole.

[odersky]

See #15885 for another possible linter task.