nvim: refactor luasnip as plugin

luasnip works better as a plugin and not a ftplugin

.config/nvim/ftplugin/go.lua

@@ -1,21 +1 @@
 vim.bo.expandtab = false
-
-local once = require("once")
-once.test_and_load("ft_go", function()
-    local luasnip = require("luasnip")
-    local s = luasnip.snippet
-    local t = luasnip.text_node
-    local cr = function() return t { "", "" } end
-    local i = luasnip.insert_node
-
-    luasnip.add_snippets("go", {
-        s("ie", {
-            t("if err != nil {"),
-            cr(),
-            t("\t"),
-            i(0),
-            cr(),
-            t("}"),
-        }),
-    })
-end)