Add diagnostics plugin and icons to Neovim configuration

hosts/totsugeki/home-manager/development/nixvim/lsp.nix

@@ -1,7 +1,21 @@
-{ pkgs, ... }: {
+{ ... }: {
   programs.nixvim.plugins = {
     lsp = {
       enable = true;
+      keymaps.lspBuf."<leader>ca" = "code_action";
+      luaConfig.post = ''
+        local signs = {
+            Error = "",
+            Warn = "",
+            Hint = "",
+            Info = ""
+        }
+
+        for type, icon in pairs(signs) do
+            local hl = "DiagnosticSign" .. type
+            vim.fn.sign_define(hl, {text = icon, texthl = hl, numhl = hl})
+        end
+      '';
       servers = {
         cssls.enable = true;
         dartls.enable = true;