feat: add detector for unused custom errors #5744

Job	Run time
tests (ubuntu-latest, 3.8, data_dependency)	33s
tests (ubuntu-latest, 3.8, path_filtering)	32s
tests (ubuntu-latest, 3.8, erc)	28s
tests (ubuntu-latest, 3.8, find_paths)	27s
tests (ubuntu-latest, 3.8, flat)	28s
tests (ubuntu-latest, 3.8, interface)	37s
tests (ubuntu-latest, 3.8, printers)	1m 15s
tests (ubuntu-latest, 3.8, slither_config)	34s
tests (ubuntu-latest, 3.8, upgradability)	53s
tests (ubuntu-latest, 3.12, data_dependency)	26s
tests (ubuntu-latest, 3.12, path_filtering)	25s
tests (ubuntu-latest, 3.12, erc)	31s
tests (ubuntu-latest, 3.12, find_paths)	26s
tests (ubuntu-latest, 3.12, flat)	31s
tests (ubuntu-latest, 3.12, interface)	31s
tests (ubuntu-latest, 3.12, printers)	1m 4s
tests (ubuntu-latest, 3.12, slither_config)	27s
tests (ubuntu-latest, 3.12, upgradability)	51s
	10m 59s

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