elixir: Disable Emmet by default (#52769)

The Emmet extension is enabled for Elixir files (`.ex`) because of the
`~H` sigil, but this is a bit unexpected if you work exclusively with
HEEx files (`.heex`) or you don't actually use Phoenix in any capacity

With this change, the user must explicitly opt into Emmet for Elixir
files, just like Tailwind

Release Notes:

- N/A

assets/settings/default.json

@@ -2042,7 +2042,7 @@
       "language_servers": ["elixir-ls", "!expert", "!next-ls", "!lexical", "..."],
     },
     "Elixir": {
-      "language_servers": ["elixir-ls", "!expert", "!next-ls", "!lexical", "..."],
+      "language_servers": ["elixir-ls", "!expert", "!next-ls", "!lexical", "!emmet-language-server", "..."],
     },
     "Elm": {
       "tab_size": 4,