Reapply "Use supermaven, not copilot"

This reverts commit 76d5017127.

lua/custom/plugins/copilot.lua

@@ -1,3 +0,0 @@
-return {
-  'github/copilot.vim',
-}

lua/custom/plugins/supermaven.lua

@@ -0,0 +1,9 @@
+-- supermaven
+-- https://github.com/supermaven-inc/supermaven-nvim
+
+return {
+  'supermaven-inc/supermaven-nvim',
+  config = function()
+    require('supermaven-nvim').setup {}
+  end,
+}