Use mix as default formatter for elixir ecosystem

Author: TheDDMbassy

lua/plugins/conform.lua

@@ -30,6 +30,10 @@ return { -- Autoformat
     end,
     formatters_by_ft = {
       lua = { 'stylua' },
+      elixir = { 'mix' },
+      eelixir = { 'mix' },
+      heex = { 'mix' },
+      surface = { 'mix' },
       -- Conform can also run multiple formatters sequentially
       -- python = { "isort", "black" },
       --