Identify unintended assignment in condition #294

This is an idea for a rule that targets the same error type as #163, an unintended assignment in a condition (instead of equality comparison).

I think the only valid use case for an assignment in a condition is for eliminating a Nil type from an expression. For that purpose, it's assigned to a local variable and the type restriction excluding Nil applies inside the block.

So we could use a couple of indicators which identify this use case:

1. The assignment target is a fresh local variable.
2. The assignment target is used in the block or subsequent parts of the condition.
3. The assigned value is not a constant or literal.
4. The assigned value should not be a local variable (you don't need to assign it again, the type restriction would apply on the original already).
5. The conditional expression is not a trailing one (because there's no body with a type restriction; I suppose it could be valid if another part of the condition uses the restricted variable, but that's pretty certainly not a good style for a trailing conditional).

There might be more indicators.

The original example from #163 would already violate 1), 2), 3) and 5) 😎

var = 1

puts "foo" if var = 1

I think this approach would be far superior to using Yoda conditionals for this use case.