It's impossible to get functions from config using nested names

[LukaszGudel]

📝 Provide detailed reproduction steps (if any)

1. Add a custom object to the editor config with functions (see the snippet below).
2. Call editor.config.get() with a nested name to get such functions from the config.

// Example config:
custom: {
	fn: () => 'foo',
	promise: async () => 'bar',
	string: 'test',
	number: 5,
	obj: {
		foo: true,
		bar: false,
		baz: () => 'foobar'
	}
}


// Get config:
console.log( editor.config.get( 'custom.fn' ) );
console.log( editor.config.get( 'custom.promise' ) );
console.log( editor.config.get( 'custom.string' ) );
console.log( editor.config.get( 'custom.number' ) );
console.log( editor.config.get( 'custom.obj' ) );
console.log( editor.config.get( 'custom.obj.baz' ) );

✔️ Expected result

Calling config.get() with the nested name should always return the element from the config.

❌ Actual result

If the nested name points to a function it returns an empty object instead :

(console.logs used above are the same as provided in the reproduction steps)

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