Cursor pagination helpers do not follow descending ordering

[kimroen]

The pagination helpers on a query such as startAt, endAt, startAfter and endAfter do not respect the ordering direction.

An example using startAfter:

const firestore = new MockFirestore().firestore();

await firestore.collection('my-collection').doc('first-document').set({ score: 1 });
await firestore.collection('my-collection').doc('second-document').set({ score: 2 });
await firestore.collection('my-collection').doc('third-document').set({ score: 3 });

// This should return second-document, then first-document, but it does not
await firestore
  .collection('my-collection')
  .orderBy('score', 'desc')
  .startAfter(3)
  .get();

In filterByCursor, the value is compared using >:

mock-cloud-firestore/src/utils/query/index.js

Lines 13 to 15 in bbe65d9

	} if (cursor === 'startAfter') {
	return data[id][prop] > value;
	}