Add a CI check that custom errors are not declared multiple times through the repo.

[Amxx]

Guidelines introduces in #4261 says:

Custom error names should not be declared twice along the library to avoid duplicated identifier declarations when inheriting from multiple contracts.

We should have an automated check for that.

[mw2000]

@Amxx Assuming we should have some sort of shell script that the CI pipeline will call which does this check?