Add Custom Sky generator for Sky Aesthetics (#667)

* Added Custom Sky generator

* Added 1.21.1 support

* fix config

* revert config change

* Fix assets config not working

* update url

* Fix merge conflict

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Co-authored-by: Misode <misoloo64@gmail.com>

src/locales/en.json

@@ -96,6 +96,7 @@
   "generator.neoforge:structure_modifier": "Structure Modifier",
   "generator.not_found": "Cannot find generator \"%0%\"",
   "generator.ohthetreesyoullgrow:configured_feature": "OTTYG Feature",
+  "generator.sky_aesthetics:sky": "Custom Sky",
   "generator.pack_mcmeta": "pack.mcmeta",
   "generator.painting_variant": "Painting Variant",
   "generator.post_effect": "Post Effect",
@@ -187,6 +188,7 @@
   "partner.neoforge": "NeoForge",
   "partner.obsidian": "Obsidian",
   "partner.ohthetreesyoullgrow": "Oh The Trees You'll Grow",
+  "partner.sky_aesthetics": "Sky Aesthetics",
   "presets": "Presets",
   "preview": "Visualize",
   "preview.auto_scroll": "Auto scroll",