Fix: Only consider functions with a single argument as function-style rules

[bmish]

I did some further real-world testing and found another heuristic we can use to ensure we only detect actual function-style rules (reducing false positives).

All function-styles rules should have the single context argument. This is needed so the rule can use context.report(...) to report violations.

Note that while we are now checking for a single argument, we are not checking anything about this argument, as it could have different names (doesn't have to be named context) or could use destructuring if desired.

This idea was also suggested here: #81 (comment)

Fixes #81.

Part of v4 release (#120).

https://eslint.org/docs/developer-guide/working-with-rules-deprecated

[aladdin-add]

LGTM, thanks!