This PR makes RuboCop::AST::IfNode support then?, similar to RuboCop::AST::WhenNode and RuboCop::AST::InNode.

This PR fixes the following incorrect autocorrect for Style/IfWithSemicolon when using multi value assignment in if with a semicolon is used:

$ echo 'if foo; bar, baz = qux else quux end' | bundle exec rubocop --stdin ...

This PR tweaks the context of "new Rails applications" in the README to reflect the release of Rails 8.0 and above.