Add Copilot Chat plugin

lua/custom/plugins/copilot-chat.lua

@@ -0,0 +1,12 @@
+return { -- https://github.com/CopilotC-Nvim/CopilotChat.nvim
+  {
+    'CopilotC-Nvim/CopilotChat.nvim',
+    dependencies = {
+      { 'nvim-lua/plenary.nvim', branch = 'master' },
+    },
+    build = 'make tiktoken',
+    opts = {
+      -- See Configuration section for options
+    },
+  },
+}