Disallow else blocks after return statements in if statements

Removes the unnecessary else when every branch of the preceding if/else if chain ends in a return, unwrapping the else body into the enclosing statement list. For example, if (x) { return y; } else { return z; } becomes if (x) { return y; } return z;. Replicates the autofix of the ESLint rule no-else-return under its default options.