Use CurseForge Fabric support instead of Jumploader

[MarioAndWeegee3]

CurseForge added native Fabric support recently, so Jumploader is no longer needed for CurseForge packs.

Proposed changes:

• --loader fabric uses a Fabric instance rather than Jumploader
• Possibly an option to upgrade existing packs using Jumploader to native Fabric

[maradotwebp]

Sure! I'll get on that right away.

[maradotwebp]

Fixed! Grab the v1.2.0 release.

To create a new fabric modpack:

• ./pax init (no change)

To migrate from existing packs:

• ./pax remove jumploader
• ./pax version <mc-version> --loader fabric