Use ESLint to ensure that exports are sorted alphabetically

There's built-in ESLint rule, see `sort-imports`, to ensure that all `import`-statements are sorted alphabetically, since that often helps with readability.
Unfortunately there's no corresponding rule to sort `export`-statements alphabetically, however there's an ESLint plugin which does this; please see https://www.npmjs.com/package/eslint-plugin-sort-exports

The only downside here is that it's not automatically fixable, but the re-ordering is a one-time "cost" and the plugin will help maintain a *consistent* ordering of `export`-statements in the future.
*Note:* To reduce the possibility of introducing any errors here, the re-ordering was done by simply selecting the relevant lines and then using the built-in sort-functionality of my editor.

src/core/image_utils.js

@@ -250,10 +250,10 @@ class GlobalImageCache {
 }
 
 export {
-  LocalImageCache,
+  GlobalImageCache,
   LocalColorSpaceCache,
   LocalFunctionCache,
   LocalGStateCache,
+  LocalImageCache,
   LocalTilingPatternCache,
-  GlobalImageCache,
 };