Add Steam Share plugin

[wopln]

Summary

This pull request adds Steam Share to the Millennium Plugin Database.

Repository:
https://github.com/wopln/Steam-Share

Features

• Adds a native Share button to Steam Store game pages.
• Allows selecting one or multiple Steam friends.
• Sends the current game's Steam Store link directly through Steam Chat.
• Matches Steam's native interface and styling.

Thank you for your review!