Add detection for Unused User-Defined Error types

[brianblank]

Describe the desired feature

If there exists a user-defined error type such as "Unauthorized" in the below example:

`// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.4;

contract VendingMachine {
error Unauthorized(); // WARNING HERE ... DEFINED BUT NOT USED
address payable owner = payable(msg.sender);

function withdraw() public {
    owner.transfer(address(this).balance);
}
// ...

}`

the expectation is that the user-defined error type should either be leveraged somewhere in the code (see below example), or the user-defined error type should be removed from the code.

`// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.4;

contract VendingMachine {
error Unauthorized();
address payable owner = payable(msg.sender);

function withdraw() public {
    if (msg.sender != owner)
        revert Unauthorized();   // WARNING GOES AWAY AS IT IS NOW USED

    owner.transfer(address(this).balance);
}
// ...

}`

The thought process is that if a user-defined error was defined but not used, then it's possible the developer missed insertion of a feature they intended to use.

This feature should work for also work for user-defined errors with parameters (the above example is of a user-defined error without parameters). An example of a user-defined error type with and without parameters can be found in the solidity documentation here: https://soliditylang.org/blog/2021/04/21/custom-errors/ .

It's possible that a user-defined error type can be used more than once, but slither should show a warning when it is never used.

Also note that solidity allows the user-defined error to be defined inside of , or outside of the contract. If the user-defined error is in a solidity file and defined outside of the contract, it should be used in that file, or there should be a way to silence the warning for files that are intended to be used as inclusions to other files.