Implement magic trailing comma for collections

This gives users a bit more control over whether things get formatted
wide or tall. This matters in practice, as Black learned the hard way,
and I did as well in the GitHub Actions example.

examples/github_actions.rcl

@@ -1,13 +1,19 @@
 {
   name = "Build",
 
-  on = { push = { branches = ["master"] }, workflow_dispatch = {} },
+  on = {
+    push = { branches = ["master"] },
+    workflow_dispatch = {},
+  },
 
   jobs = {
     "Build": {
       runs-on = "ubuntu-22.04",
       steps = [
-        { name = "Checkout", uses = "actions/checkout@v3.5.3" },
+        {
+          name = "Checkout",
+          uses = "actions/checkout@v3.5.3",
+        },
         {
           name = "Install Nix",
           uses = "cachix/install-nix-action@v18",