Implement support for flat collections in Mixed

[elle-j]

What, How & Why?

Adds support for storing flat lists and dictionaries as the underlying value of a Mixed property on non-synced realms. Hence, the list or dictionary can contain any of the previously allowed Mixed values, but not another nested collection. Nested lists and dictionaries will be implemented as a separate PR.

Sets are not supported as a Mixed value.

Notes

• Tests for the array methods on List and OrderedCollection will be added as part of the nested collections implementation.
• Core branch used.

Brief overview of usage

class CustomObject extends Realm.Object {
  value!: Realm.Types.Mixed;

  static schema: ObjectSchema = {
    name: "CustomObject",
    properties: {
      value: "mixed",
    },
  };
}

const realm = await Realm.open({ schema: [CustomObject] });

// Create an object with a list value as the Mixed property.
const realmObject = realm.write(() => {
  return realm.create(CustomObject, { value: [1, "hello", true] });
});

// Get all objects with a list as the Mixed `value` property
// that contains an item matching `"hello"` at index 1.
realm.objects(CustomObject).filtered(`value[1] == $0`, "hello");

// Update the Mixed property to a dictionary.
realm.write(() => {
  realmObject.value = { int: 1, string: "hello", bool: true };
});

// Get all objects with a dictionary as the Mixed `value`
// property that has a key `string` with the value `"hello"`.
realm.objects(CustomObject).filtered(`value.string == $0`, "hello");

This closes #6342 and #6343.

☑️ ToDos

• 📝 Changelog entry (could add when the feature branch is merged into main)
• 🚦 Tests

[coveralls-official]

coverage: 85.931% (+0.3%) from 85.663%
when pulling efdb3d3 on lj/flat-collections-in-mixed
into 276dc67 on lj/collections-in-mixed.

[kneth]

It is amazing to see that collections in mixed is coming to JS 😍

[takameyer]

Great! LGTM

[kraenhansen]

I like to see this much test vs implementation code. Great job!