overlays/iosevka: refactor

iosevka overlay now uses iosevka config build plans in extesible forms!

nix/overlays/iosevka/spacings.nix

@@ -0,0 +1,24 @@
+{ lib }:
+
+let
+  list = [
+    { spacing = "normal"; }
+
+    # Iosevka + Ghostty makes "…" display in a odd way.
+    # We use "term" width to avoid this.
+    # https://github.com/ghostty-org/ghostty/discussions/10335
+    { spacing = "term"; }
+  ];
+in
+
+map (
+  { spacing }@cfg:
+  let
+    name = lib.toSentenceCase spacing;
+  in
+  final: prev:
+  cfg
+  // {
+    family = prev.family + "-${name}";
+  }
+) list