fix(react-modal): animation for backdrop

[cyanxiao]

📝 Description

Add a brief description

Original backdrop didn't perform transition or animation when it shows up. I did find that there are some transition codes in StyledBackdropLayer but they didn't come into effect because there was no change in opacity after backdrop had been rendered.

Please describe the current behavior that you are modifying

Modal is set to invisible for illustration purpose.

Before:

After:

🚀 New behavior

Please describe the behavior or changes this PR adds

Add fade-in effect for backdrop. It applies to opacity & backdrop filter.

💣 Is this a breaking change (Yes/No): No

📝 Additional Information