feat(relation): #2 Implements relation transformer

(cherry picked from commit 57f361e)
closes #3, resolves #2

src/transformers/field-relation.transform.ts

@@ -2,9 +2,12 @@ import type { CmsFieldBase, CmsFieldRelation } from 'decap-cms-core';
 
 import type { Transformer } from '../utils/transform.utils.js';
 
-// TODO implement transform relations
-// https://decapcms.org/docs/widgets/#relation
-export const transformRelationField: Transformer<CmsFieldBase & CmsFieldRelation> = () => ({
-  compiled: 'z.string()',
-  dependencies: ['z'],
-});
+export const transformRelationField: Transformer<CmsFieldBase & CmsFieldRelation> = ({
+  collection,
+  multiple = false,
+}) => {
+  return {
+    compiled: multiple ? `z.array(reference('${collection}'))` : `reference('${collection}')`,
+    dependencies: ['z', 'reference'],
+  };
+};