fix: export entity-reference positioner (#1781)

There is a positioner for entity-reference-extension but it's not properly exported. This makes it difficult to use.

See here an example from annotation-extension that does export the positioner: https://github.com/remirror/remirror/blob/1bc49d747b693146bb320017de506aa92efcbc74/packages/remirror__extension-annotation/src/index.ts#L2

packages/remirror__extension-entity-reference/src/index.ts

@@ -1,3 +1,4 @@
 export * from './entity-reference-extension';
+export { centeredEntityReferencePositioner } from './entity-reference-positioners';
 export type { EntityReferenceMetaData } from './types';
 export { findMinMaxRange } from './utils/ranges';