custom-error-definition: Cover exports name

[MrHen]

Fixes #190.

I cheated a bit for this one. If the rule is turned on it will force the error name to be correct so the rule only has to check for a mismatch on the exports identifier in the case where the right hand side is an error class.

There was another interesting case that I did NOT cover:

class FooError extends Error {
	constructor(error) {
		super(error);
		this.name = 'FooError';
	}
};
exports.fooError = FooError;

It would theoretically be possible to find an error definition from earlier in the same file. If we want this scenario covered, then I can add it in.

[sindresorhus]

It would theoretically be possible to find an error definition from earlier in the same file. If we want this scenario covered, then I can add it in.

Nah. Not worth the effort since we'll have ESM in Node.js soon.