tell vscode, if formatting, use whatever our trunk formatter wants (#4186)

without this flag if the user has set some other formatter (clang)
in their user level settings, it will be looking in the wrong directory
for the clang options (we want the options in .trunk/clang)

Note: formatOnSave is true in master, which means a bunch of our older
files are non compliant and if you edit them it will generate lots of
formatting related diffs.  I guess I'll start letting that happen with
my future commits ;-).

.vscode/settings.json

@@ -4,5 +4,8 @@
   "trunk.enableWindows": true,
   "files.insertFinalNewline": false,
   "files.trimFinalNewlines": false,
-  "cmake.configureOnOpen": false
+  "cmake.configureOnOpen": false,
+  "[cpp]": {
+    "editor.defaultFormatter": "trunk.io"
+  }
 }