Cache symbol completion result #4031

Job	Run time
lint	13s
Debug compatibility test	5m 45s
ruby-versions / Generate Ruby versions	3s
rake test 2.7 (latest reline)	53s
vterm-yamatanooroti 2.7 (latest reline)	47s
rake test 2.7	52s
vterm-yamatanooroti 2.7	47s
rake test 3.0 (latest reline)	42s
vterm-yamatanooroti 3.0 (latest reline)	28s
rake test 3.0	48s
vterm-yamatanooroti 3.0	27s
rake test 3.1 (latest reline)	42s
vterm-yamatanooroti 3.1 (latest reline)	30s
rake test 3.1	46s
vterm-yamatanooroti 3.1	31s
rake test 3.2 (latest reline)	46s
vterm-yamatanooroti 3.2 (latest reline)	29s
rake test 3.2	48s
vterm-yamatanooroti 3.2	31s
rake test 3.3 (latest reline)	42s
vterm-yamatanooroti 3.3 (latest reline)	1m 3s
rake test 3.3	40s
vterm-yamatanooroti 3.3	34s
rake test head (latest reline)	54s
rake test head	55s
vterm-yamatanooroti head (latest reline)	31s
rake test truffleruby-head (latest reline)	2m 51s
vterm-yamatanooroti head	1m 3s
rake test truffleruby-head	2m 49s
	28m 50s

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