PERF: Improve theme stylesheet compilation performance (#12850)

When building the `scss_load_paths`, we were creating a full export of the theme (including uploads), and not cleaning it up. With many uploads, this can be extremely slow (because it downloads every upload from S3), and the lack of cleanup could cause a disk to fill up over time.

This commit updates the ZipExporter to provide a `with_export_dir` API, which takes care of cleanup. It also adds a kwarg which allows exporting only extra_scss fields. This should make things much faster for themes with many uploads.

lib/theme_store/zip_exporter.rb

@@ -25,11 +25,22 @@ class ThemeStore::ZipExporter
     FileUtils.rm_rf(@temp_folder)
   end
 
-  def export_to_folder
+  def with_export_dir(**kwargs)
+    export_to_folder(**kwargs)
+
+    yield File.join(@temp_folder, @export_name)
+  ensure
+    cleanup!
+  end
+
+  private
+
+  def export_to_folder(extra_scss_only: false)
     destination_folder = File.join(@temp_folder, @export_name)
     FileUtils.mkdir_p(destination_folder)
 
     @theme.theme_fields.each do |field|
+      next if extra_scss_only && !field.extra_scss_field?
       next unless path = field.file_path
 
       # Belt and braces approach here. All the user input should already be
@@ -50,19 +61,16 @@ class ThemeStore::ZipExporter
       File.write(path, content)
     end
 
-    File.write(File.join(destination_folder, "about.json"), JSON.pretty_generate(@theme.generate_metadata_hash))
+    if !extra_scss_only
+      File.write(
+        File.join(destination_folder, "about.json"),
+        JSON.pretty_generate(@theme.generate_metadata_hash)
+      )
+    end
 
     @temp_folder
   end
 
-  def export_dir
-    export_to_folder
-
-    File.join(@temp_folder, @export_name)
-  end
-
-  private
-
   def export_package
     export_to_folder