prefer-early-return doesn't work when there is a variable

[BenjaminVanRyseghem]

Overview

When extracting the if condition into a variable, the rule is suddenly satisfied.

Not sure if it's a bug or not, but this feel like it could be at least improved (yet I don't know if it's really feasible)

Examples:

The following is considered warning

	foo() {
		if (a) {
			b();
			c();
		}
	}

yet, the following is not

	foo() {
        var bar = a;
		if (bar) {
			b();
			c();
		}
	}

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Checklist

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[WesleyYue]

This also doesn't work:

export function foo1(a: string) {
  if (a) {
    console.log(a);
  }
}