Enforce explicit function return types

This re-enables the `@typescript-eslint/explicit-module-boundary-types`
check and also enables the
`@typescript-eslint/explicit-function-return-type` check.

Signed-off-by: David Mehren <git@herrmehren.de>

src/media/backends/filesystem-backend.ts

@@ -63,7 +63,7 @@ export class FilesystemBackend implements MediaBackend {
     return join(this.uploadDirectory, fileName);
   }
 
-  private async ensureDirectory() {
+  private async ensureDirectory(): Promise<void> {
     try {
       await fs.access(this.uploadDirectory);
     } catch (e) {