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.

mods/distanthorizons.pw.toml

@@ -2,6 +2,11 @@ name = "Distant Horizons"
 filename = "DistantHorizons-3.3.2-1.21.1-fabric-neoforge.jar"
 side = "both"
 
+[option]
+optional = true
+default = false
+description = "Extended render distance via level-of-detail terrain (heavy on GPU/VRAM)"
+
 [download]
 url = "https://cdn.modrinth.com/data/uCdwusMi/versions/Ez3cx7Yd/DistantHorizons-3.3.2-1.21.1-fabric-neoforge.jar"
 hash-format = "sha512"