Move assignment check to RubyLex #2323

Job	Run time
ruby-versions / Generate Ruby versions	3s
vterm-yamatanooroti 2.7 (latest reline)	1m 50s
vterm-yamatanooroti 2.7	1m 12s
vterm-yamatanooroti 3.0 (latest reline)	49s
vterm-yamatanooroti 3.0	44s
vterm-yamatanooroti 3.1 (latest reline)	48s
vterm-yamatanooroti 3.1	1m 0s
vterm-yamatanooroti 3.2 (latest reline)	45s
vterm-yamatanooroti 3.2	43s
vterm-yamatanooroti head (latest reline)	43s
vterm-yamatanooroti head	49s
rake test 2.7 (latest reline)	41s
rake test 2.7	52s
rake test 3.0 (latest reline)	36s
rake test 3.0	33s
rake test 3.1 (latest reline)	31s
rake test 3.1	36s
rake test 3.2 (latest reline)	37s
rake test 3.2	29s
rake test head (latest reline)	39s
rake test head	28s
rake test truffleruby-head (latest reline)	7m 50s
rake test truffleruby-head	7m 49s
	31m 7s

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