feat(erlang): add ELP and Treesitter support

LazyVim's erlang extra still enables erlangls, but erlang_ls was
archived and nvim-lspconfig has dropped its config in favour of ELP
(LazyVim/LazyVim#7249). Configure ELP, which Mason installs, and the
erlang Treesitter parser directly.

lua/plugins/erlang.lua

@@ -0,0 +1,16 @@
+return {
+  {
+    "nvim-treesitter/nvim-treesitter",
+    opts = { ensure_installed = { "erlang" } },
+  },
+  {
+    "neovim/nvim-lspconfig",
+    opts = {
+      servers = {
+        -- LazyVim's erlang extra still enables erlangls, which is unmaintained
+        -- and gone from nvim-lspconfig. ELP replaces it.
+        elp = {},
+      },
+    },
+  },
+}