feat(iferr): add to installer & config

lua/gopher/config.lua

@@ -6,6 +6,7 @@ local M = {
       gomodifytags = "gomodifytags",
       gotests = "gotests",
       impl = "impl",
+      iferr = "iferr",
     },
   },
 }

lua/gopher/installer.lua

@@ -4,6 +4,7 @@ local urls = {
   gomodifytags = "github.com/fatih/gomodifytags",
   impl = "github.com/josharian/impl",
   gotests = "github.com/cweill/gotests/...",
+  iferr = "github.com/koron/iferr",
 }
 
 ---@param pkg string