feat: adds `:targetblank` scrubber #158

Job	Run time
rubocop	17s
rubocop	12s
downstream-rails-html-sanitizer	37s
downstream-rails-html-sanitizer	25s
test (2.5)	25s
test (2.5)	30s
test (2.6)	28s
test (2.6)	35s
test (2.7)	29s
test (2.7)	28s
test (3.0)	18s
test (3.0)	13s
test (3.1)	10s
test (3.1)	14s
test (3.2)	13s
test (3.2)	13s
test (head)	15s
test (head)	12s
test (truffleruby-head)	1m 22s
test (truffleruby-head)	1m 7s
test (jruby-9.4)	32s
test (jruby-9.4)	1m 2s
test (jruby-head)	34s
test (jruby-head)	1m 0s
	11m 51s

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