feat(@toss/react): enhance Separated component

[metacode22]

Overview

I'm really enjoying using this Separated component. And I've thought of a few improvements.

• Added first and last props to include separators at the beginning and end. dc41563
• Preserved child element keys instead of replacing them with indexes. 67851af
• Removed isValidElement filtering to support all ReactNode types. fa3e052
• Added test code and documentation for these changes. 8666ba4 397626a

I hope you like it. Let me know if you'd like any adjustments. Thank you!

PR Checklist

• I read and included theses actions below

1. I have read the Contributing Guide
2. I have written documents and tests, if needed.

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