feat: add detector for unused custom errors

[gvladika]

New detector looks for Solidity custom errors which are declared, but never used. This can indicate a mistake, ie. error is declared but check which would invoke the error was never put in place.

It can be executed like this:

slither .  --detect unused-custom-errors

Output will list identified unused errors and the respective file names. Ie.

The following unused error(s) should be removed:
        -UnusedLibError() (tests/e2e/detectors/test_data/unused-custom-errors/0.8.15/CustomErrorsLib.sol)

        -UnusedErrorLibConsumerErr() (tests/e2e/detectors/test_data/unused-custom-errors/0.8.15/ErrorLibConsumer.sol)

        -UnusedParentErrWithArg(uint256) (tests/e2e/detectors/test_data/unused-custom-errors/0.8.15/ContractToTestForCustomErrors.sol)

        -UnusedParentErr() (tests/e2e/detectors/test_data/unused-custom-errors/0.8.15/ContractToTestForCustomErrors.sol)

        -UnusedTopLevelErr() (tests/e2e/detectors/test_data/unused-custom-errors/0.8.15/ContractToTestForCustomErrors.sol)

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