feat(core): optionally log if entity is managed or not

```ts
process.env.MIKRO_ORM_LOG_EM_ID = '1';
// prints something like `User [managed by 3] { ... }`
console.log(user);
```

packages/core/src/entity/EntityHelper.ts

@@ -124,6 +124,16 @@ export class EntityHelper {
       const ret = inspect(object, { depth });
       let name = (this as object).constructor.name;
 
+      const showEM = ['true', 't', '1'].includes(process.env.MIKRO_ORM_LOG_EM_ID?.toString().toLowerCase() ?? '');
+
+      if (showEM) {
+        if (helper(this).__em) {
+          name += ` [managed by ${helper(this).__em.id}]`;
+        } else {
+          name += ` [not managed]`;
+        }
+      }
+
       // distinguish not initialized entities
       if (!helper(this).__initialized) {
         name = `(${name})`;

tests/EntityHelper.mongo.test.ts

@@ -205,6 +205,16 @@ describe('EntityHelperMongo', () => {
       "  baz: (FooBaz) { _id: ObjectId('5b0ff0619fbec620008d2414') }\n" +
       '}');
 
+    process.env.MIKRO_ORM_LOG_EM_ID = '1';
+    actual = inspect(bar);
+    process.env.MIKRO_ORM_LOG_EM_ID = '0';
+
+    expect(actual).toBe('FooBar [not managed] {\n' +
+      '  meta: { onCreateCalled: false, onUpdateCalled: false },\n' +
+      "  name: 'bar',\n" +
+      "  baz: (FooBaz [managed by 1]) { _id: ObjectId('5b0ff0619fbec620008d2414') }\n" +
+      '}');
+
     const god = new Author('God', 'hello@heaven.god');
     const bible = new Book('Bible', god);
     bible.createdAt = new Date('2020-07-18T17:31:08.535Z');