Add Modern Gamertag Support to Java Edition (Unicode Usernames)

Currently, Minecraft: Java Edition only supports ASCII-based usernames (A–Z, 0–9, _). Meanwhile, the Bedrock Edition already uses Microsoft’s modern gamertag system, which allows Unicode characters and international names.

I would like to suggest bringing the Modern Gamertag system to the Java Edition to achieve better parity between Java and Bedrock.

This would allow players to:
- use names in their native languages (including accents and non-Latin scripts)
- express themselves more freely
- maintain a consistent identity across all Minecraft platforms

Since authentication is already handled via Microsoft accounts, this seems like a natural next step toward unifying the ecosystem.

This would greatly improve internationalization and make Minecraft more inclusive for its global player base.

This could lay the groundwork for cross-play.