Share variable inspection logic between CDP and DAP #1708

Job	Run time
ruby-versions / Generate Ruby versions	3s
test (2.7)	1m 45s
test (3.0)	1m 53s
test (3.1)	1m 43s
test (3.2)	1m 40s
test (3.3)	1m 38s
test (debug)	2m 20s
test (head)	1m 17s
	12m 19s

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