plugin-ext: default the folderExpanded icon

In file icon themes, VS Code will use the
"folder" icon also for expanded folders
in the case that the theme omits the
"folderExpanded" entry. Theia should
do the same for consistency.

Fixes eclipse-theia#12727

Signed-off-by: Christian W. Damus <cdamus.ext@eclipsesource.com>

packages/plugin-ext/src/main/browser/plugin-icon-theme-service.ts

@@ -390,6 +390,12 @@ export class PluginIconTheme extends PluginIconThemeDefinition implements IconTh
     ): void {
         if (associations.folder) {
             accept(associations.folder, this.folderIcon);
+            if (associations.folderExpanded === undefined) {
+                // Use the same icon for expanded state (issue #12727). Check for
+                // undefined folderExpanded property to allow for
+                // "folderExpanded": null in case a developer really wants that
+                accept(associations.folder, this.folderExpandedIcon);
+            }
             this.hasFolderIcons = true;
         }
         if (associations.folderExpanded) {