Use union type instead of enum for extent corner

src/ol/extent.js

@@ -1,7 +1,6 @@
 /**
  * @module ol/extent
  */
-import Corner from './extent/Corner.js';
 import Relationship from './extent/Relationship.js';
 import {assert} from './asserts.js';
 
@@ -11,6 +10,11 @@ import {assert} from './asserts.js';
  * @api
  */
 
+/**
+ * Extent corner.
+ * @typedef {'bottom-left' | 'bottom-right' | 'top-left' | 'top-right'} Corner
+ */
+
 /**
  * Build an extent that includes all given coordinates.
  *
@@ -484,18 +488,18 @@ export function getCenter(extent) {
 /**
  * Get a corner coordinate of an extent.
  * @param {Extent} extent Extent.
- * @param {import("./extent/Corner.js").default} corner Corner.
+ * @param {Corner} corner Corner.
  * @return {import("./coordinate.js").Coordinate} Corner coordinate.
  */
 export function getCorner(extent, corner) {
   let coordinate;
-  if (corner === Corner.BOTTOM_LEFT) {
+  if (corner === 'bottom-left') {
     coordinate = getBottomLeft(extent);
-  } else if (corner === Corner.BOTTOM_RIGHT) {
+  } else if (corner === 'bottom-right') {
     coordinate = getBottomRight(extent);
-  } else if (corner === Corner.TOP_LEFT) {
+  } else if (corner === 'top-left') {
     coordinate = getTopLeft(extent);
-  } else if (corner === Corner.TOP_RIGHT) {
+  } else if (corner === 'top-right') {
     coordinate = getTopRight(extent);
   } else {
     assert(false, 13); // Invalid corner