Adds an [option] block so Distant Horizons is a user-toggleable mod instead
of always installed, matching how Controlify is already declared.
default = false expresses opt-in wherever the consuming exporter supports
it: modrinth/export.go reads only Optional, so in an mrpack the mod becomes
a toggle (that format has no off-by-default concept), while
curseforge/export.go reads Default and marks it optional-and-disabled.
Nothing in the pack depends on or conflicts with Distant Horizons, so no
other mod is left unsatisfied by it being skippable.
logan