Prefer string enum to numeric enum

String enums have the advantage that enum values are more meaningful at runtime
for debugging. See discussion at
https://hypothes-is.slack.com/archives/C1M8NH76X/p1682585801783819.

src/sidebar/markdown-commands.ts

@@ -21,8 +21,8 @@ export type EditorState = {
  * {@link convertSelectionToLink}.
  */
 export enum LinkType {
-  ANCHOR_LINK,
-  IMAGE_LINK,
+  ANCHOR_LINK = 'ANCHOR_LINK',
+  IMAGE_LINK = 'IMAGE_LINK',
 }
 
 /**