plugins/formatter: use latest edition for rustfmt

plugins/formatter/neoformat.lua

@@ -1,6 +1,11 @@
 vim.g.neoformat_try_node_exe = 1
 vim.g.neoformat_only_msg_on_error = 1
 vim.g.neoformat_enabled_markdown = { "denofmt" }
+vim.g.neoformat_rust_rustfmt = {
+	exe = "rustfmt",
+	args = { "--edition 2021" },
+	stdin = 1,
+}
 
 vim.cmd([[
 aug fmt