Add parsePackage method, bump dependencies, target Node 16

[tommy-mitchell]

Closes #28.

Adds a new parsePackage method that operates on objects and strings:

const package_ = parsePackage({name: 'unicorn', version: '1.0.0'});
//=> NormalizedPackageJson

const package_ = parsePackage('{"name": "unicorn", "version": "1.0.0"}', {normalize: false});
//=> PackageJson

I've bumped the dependencies and the minimum Node.js version as well, so this is a breaking change. With the AVA version bump, I've changed to test script so that we don't have to pass --no-worker-threads and use process.chdir():

"test": "xo && tsd && cd test && ava"

[tommy-mitchell]

Current checks:

const isObject = packageFile !== null && typeof packageFile === 'object' && !Array.isArray(packageFile);
const isString = typeof packageFile === 'string';

if (!isObject && !isString) {
	throw new TypeError('`packageFile` should be either an `object` or a `string`.');
}

// Input should not be modified - if `structuredClone` is available, do a deep clone, shallow otherwise
const clonedPackageFile = isObject
	? (globalThis.structuredClone === undefined
		? {...packageFile}
		: structuredClone(packageFile))
	: packageFile;

[tommy-mitchell]

@sindresorhus whoops, left in a typo (trailing slash):