Make `Node#left_curly_brace?` aware of lambda brace #1003

Job	Run time
Specs \| 2.6	7s
Coding Style \| 3.0	20s
Specs \| 2.7	2s
Main Gem Specs \| RuboCop: master \| 2.7	39s
Specs \| 3.0	1s
Main Gem Specs \| RuboCop: master \| 3.0	39s
Specs \| 3.1	1s
Main Gem Specs \| RuboCop: v1.35.0 \| 3.2	39s
Specs \| 3.2	1s
Specs \| head	1s
Specs \| mingw	1s
Coverage \| 2.6	38s
Specs "modern" \| 3.0	12s
Specs \| jruby	18s
Specs \| truffleruby	16s
Specs \| truffleruby-head	20s
	4m 15s

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