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")

stdlib/stdlib.gradle.kts

@@ -17,8 +17,8 @@ plugins {
   pklAllProjects
   base
   `maven-publish`
-  id("com.diffplug.spotless")
   pklPublishLibrary
+  pklSpotlessFormat
   signing
 }
 
@@ -51,13 +51,3 @@ publishing {
 }
 
 signing { sign(publishing.publications["stdlib"]) }
-
-spotless {
-  format("pkl") {
-    target("*.pkl")
-    licenseHeaderFile(
-      rootProject.file("buildSrc/src/main/resources/license-header.line-comment.txt"),
-      "/// ",
-    )
-  }
-}