fix: export Field-related types from Blockly (#6877)

* chore: re-exported Field config types from Blockly

* mark `@internal` types, export field error, and add validator comments

core/field.ts

@@ -48,8 +48,6 @@ import type {WorkspaceSvg} from './workspace_svg.js';
  * A function that is called to validate changes to the field's value before
  * they are set.
  *
- * **NOTE:** Validation returns one option between `T`, `null`, and `undefined`.
- *
  * @see {@link https://developers.google.com/blockly/guides/create-custom-blocks/fields/validators#return_values}
  * @param newValue The value to be validated.
  * @returns One of three instructions for setting the new value: `T`, `null`,
@@ -1315,6 +1313,8 @@ export interface FieldConfig {
 /**
  * For use by Field and descendants of Field. Constructors can change
  * in descendants, though they should contain all of Field's prototype methods.
+ *
+ * @internal
  */
 export type FieldProto = Pick<typeof Field, 'prototype'>;