Allow overloading of custom errors

## Abstract

Allow custom error with the same name, but different parameter, to be declared

## Motivation

According to [this blog post](https://blog.soliditylang.org/2021/04/21/custom-errors/), custom errors are encode similarly to function call.

> The error data would be encoded identically as the ABI encoding for function calls, i.e.,
> `abi.encodeWithSignature("InsufficientBalance(uint256,uint256)", balance[msg.sender], amount)`.

However, declaring
```
error InsufficientBalance(address owner); // for ERC20
error InsufficientBalance(address owner, uint256 tokenId); // for ERC1155
```
result in an error `DeclarationError: Identifier already declared`.

This should be allowed IMO.

## Backwards Compatibility

N/A

No source files, tests, or compiler entry points are named in the issue. Start by locating the handling of custom-error declarations and duplicate identifiers, then find tests covering error declarations; done means same-name custom errors with distinct parameter lists are accepted.

Written by the indexing model from the issue text.