feat: a formatter for SML (Standard ML) (#353)

Co-authored-by: Dennis Xu <[email protected]>

Author: denx20

lua/conform/formatters/smlfmt.lua

@@ -0,0 +1,8 @@
+---@type conform.FileFormatterConfig
+return {
+  meta = {
+    url = "https://github.com/shwestrick/smlfmt",
+    description = "A custom parser and code formatter for Standard ML.",
+  },
+  command = "smlfmt",
+}