Enforce Pkl formatting of stdlib with spotless (#1253)

This adds a spotless formatting step using the new pkl formatter.

This only formats Pkl sources in the stdlib, because other sources
are possibly test input and not meant to be formatted.

stdlib/base.pkl

@@ -437,7 +437,7 @@ class RenderDirective {
   text: String
 
   /// The bytes to output as-is (without escaping or quoting).
-  /// 
+  ///
   /// Only used by [BytesRenderer], ignored by [ValueRenderer].
   @Since { version = "0.30.0" }
   bytes: Bytes = text.encodeToBytes("UTF-8")