feat(pack): add Icy's Better Horses and its Modonomicon dependency

Icy's Better Horses declares client_side/server_side as required, so it ships
to both sides. It hard-requires Modonomicon [1.120.4,) on BOTH sides, which the
pack was missing; GeckoLib [4.7.5,) is already present at 4.9.3, and
cloth-config (optional) is already present too.

Pinned to the NeoForge 1.21.1 builds: Modrinth lists two different files both
numbered 2.1.0.1 (Forge/1.20.1 and NeoForge/1.21.1), and resolving by version
number alone silently picks the wrong one.

Also ignore shaderpacks/*.zip and xaero-migration-kit/ in .packwizignore. The
former duplicated the existing .pw.toml metafiles and the latter is local
migration scratch that the file already says must never ship, but packwiz
refresh was indexing both into the pack.

mods/icys-better-horses.pw.toml

@@ -0,0 +1,13 @@
+name = "Icy's Better Horses"
+filename = "icys-better-horses-2.1.0.1.jar"
+side = "both"
+
+[download]
+url = "https://cdn.modrinth.com/data/XlUm5I57/versions/I0I1AE66/icys-better-horses-2.1.0.1.jar"
+hash-format = "sha512"
+hash = "208d1345e44444f476515bb7b7196966035b2c03ef238946fe059eac9d038463eb59de2ee8883b01f550ded2b1fc959c9b76b1c66cb063ea865c3e38f6b15f99"
+
+[update]
+[update.modrinth]
+mod-id = "XlUm5I57"
+version = "I0I1AE66"