Change priority order of ignorePath and ignorePatterns (or at least document it)

[alexeyr-ci]

Feature request description

Currently the ignorer is generated as follows:

depcheck/src/index.js

Lines 62 to 90 in 77fe0e2

	function getIgnorer({ rootDir, ignorePath, ignorePatterns }) {
	const ignorer = ignore();
	ignorer.add(ignorePatterns);
	// If an .*ignore file is configured
	if (ignorePath) {
	const ignorePathFile = path.resolve(rootDir, ignorePath);
	if (fs.existsSync(ignorePathFile)) {
	debug('depcheck:ignorer')(`Using ${ignorePathFile} as ignore file.`);
	const ignorePathFileContent = fs.readFileSync(ignorePathFile, 'utf8');
	ignorer.add(ignorePathFileContent);
	}
	return ignorer;
	}
	// Fallback on .depcheckignore or .gitignore
	const ignoreFile = ['.depcheckignore', '.gitignore']
	.map((file) => path.resolve(rootDir, file))
	.find((file) => fs.existsSync(file));
	if (ignoreFile) {
	debug('depcheck:ignorer')(`Using ${ignoreFile} as ignore file.`);
	const ignoreContent = fs.readFileSync(ignoreFile, 'utf8');
	ignorer.add(ignoreContent);
	}
	return ignorer;
	}

I'd prefer ignorePatterns to be added after ignorePath content. The use-case is to "unignore" generated code which may be the only user of some dependencies. Failing that, document:

1. That ignorePath follows ignorePatterns;
2. The fallback files if ignorePath is not given.

Also, if ignorePath is explicitly specified and doesn't exist, that should probably be an error.

I'd be happy to provide a PR.

Code snippets (if applicable)

Not applicable.

Any extra info

Anything else you want to include. Thank you for your input!