Add Lint/NotNil rule

[Sija]

Resolves #284

[veelenga]

Lgtm

[Blacksmoke16]

@Sija @veelenga Do we think this is really something we should have enabled by default? I don't think a blanket "not_nil! is bad" without taking the context into consideration, which a linter is unable to do, is providing anything real meaningful.

Maybe for someone new to the language it might sway them to do something different, but then everyone else who understands when/how to use it is forced to either go thru their code and add ameba:disable Lint/NotNil everywhere, or disable the rule entirely themselves.

E.g. https://github.com/athena-framework/athena/actions/runs/3398343461/jobs/5651304890. I'd rather not go thru all of these cases to properly handle nil just to make ameba happy when I know something the compiler doesn't.

[Sija]

@Blacksmoke16 I was hesitant myself at the beginning, yet after checking this against several repos (incl. crystal), most of the issues were legitimate refactoring opportunities - and IMO these would be:

• missing assignment to a local var: @foo.not_nil!
• missing proper error handling: foo.not_nil! vs raise ... unless foo

Remaining ones, incl. cases in which using not_nil! is the only sensible thing to do, turned out to be a small minority, easily disabled with # ameba:disable pragma - and I regard this as a positive thing, since you're giving by-watchers a signal about your intentions.

All in all, using not_nil! is a code-smell, so all of the cases should be carefully decided upon, IMO even at the cost of potential brief annoyance.