Make `RuboCop::AST::IfNode` support `then?` #1234

Job	Run time
Coding Style \| 3.0	42s
Specs \| 2.7	40s
Specs \| 3.0	35s
Prism	23s
Main Gem Specs \| RuboCop: master \| 2.7	2m 7s
Specs \| 3.1	30s
Main Gem Specs \| RuboCop: master \| 3.0	1m 43s
Specs \| 3.2	32s
Main Gem Specs \| RuboCop: v1.61.0 \| 3.3	2m 11s
Specs \| 3.3	30s
Specs \| head	27s
Coverage \| 2.7	48s
Specs "modern" \| 3.0	43s
Specs \| jruby	46s
Specs \| jruby-head	42s
Specs \| truffleruby	53s
Specs \| truffleruby-head	1m 5s
	15m 17s

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