chore(pack): mark Distant Horizons as an optional mod

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.

index.toml

@@ -212,7 +212,7 @@ metafile = true
 
 [[files]]
 file = "mods/distanthorizons.pw.toml"
-hash = "6d3e312dbf8db12dd1b6fb3bd1d29f210ab48024a0b2ebdf02fd32c265955c5d"
+hash = "7fd6d5b01ee6f20585b0a0b704f54c9f05772381bdbcbde30538452514774eb9"
 metafile = true
 
 [[files]]