add macOS support

[yueyuzhao]

Resolves

• Resolves enable ScratchJr to run on MacOS #379

Proposed Changes

1. remove Firebase for macOS
2. change CONTENTS_FOLDER_PATH to UNLOCALIZED_RESOURCES_FOLDER_PATH in Run Script

Reason for Changes

1. Firebase does not support macOS, see Firebase;
2. When calling [[NSBundle mainBundle] pathForResource], it will look for resource in the Resources folder, but we copied the HTML5 resources to the /Contents folder. For macOS, $CONTENTS_FOLDER_PATH is $WRAPPER_NAME/Contents and $UNLOCALIZED_RESOURCES_FOLDER_PATH is
   $CONTENTS_FOLDER_PATH/Contents/Resources, and $UNLOCALIZED_RESOURCES_FOLDER_PATH works fine for both macOS and iOS. See Build Setting Reference from Apple

Test Coverage

• macOS Catalina 10.15.6

[yueyuzhao]

Things that should be checked and done:

• Camera: taking picture
• Recording: record and play sound
• Exporting project
• Importing project
• Window resize?