feat: add CopilotChat.nvim

Actually, I've had this for a while but chose not to commit. Still unsure if
I'm going to keep it as I don't use it at all

.config/nvim/lua/plugins/copilot-chat.lua

@@ -0,0 +1,15 @@
+return {
+   {
+      "CopilotC-Nvim/CopilotChat.nvim",
+      dependencies = {
+         { "zbirenbaum/copilot.lua" },
+         { "nvim-lua/plenary.nvim", branch = "master" }, -- for curl, log and async functions
+      },
+      build = "make tiktoken",                        -- Only on MacOS or Linux
+      cmd = "CopilotChat",
+      opts = {
+         -- See Configuration section for options
+      },
+      -- See Commands section for default commands if you want to lazy load on them
+   },
+}