Allow modpacks to include custom JVM arguments in .mrpack files

[KdGaming0]

Please confirm the following.

• I checked the existing issues for duplicate feature requests
• I have checked that this feature request is not on our roadmap

What parts of Modrinth is your feature request related too?

App

Is your suggested feature related to a problem? Please describe.

Some modpacks are large and can benefit from specific JVM arguments to improve performance or stability.
For example:

• Packs with high-resolution or large texture packs often need increased memory or special JVM flags to prevent timeouts.
• Performance-heavy modpacks can run smoother with tuned JVM arguments.

Right now, modpack creators cannot ship JVM arguments directly with their pack. Users need to set them up manually, which can be confusing for less experienced players and lead to poor performance or crashes if skipped.

Describe the solution you'd like

Allow .mrpack files to include a section for custom JVM arguments.
When a user creates an instance from a modpack, these arguments would automatically apply in the launcher for that instance.
The launcher should still allow users to edit or override these arguments if they wish.

Describe alternatives you've considered

• Asking users to manually add JVM arguments in their launcher settings.
  Downside: Not user-friendly, and often overlooked.

Additional context

No response