Add internal intellij plugin (#1248)

This introduces an IntelliJ plugin that's meant to assist with development of the Pkl codebase itself.

The plugin adds a file editor that opens snippet tests in a split editor pane, showing the input on the left side and output on the right side.

pkl-internal-intellij-plugin/pkl-internal-intellij-plugin.gradle.kts

@@ -0,0 +1,35 @@
+/*
+ * Copyright © 2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+plugins {
+  pklAllProjects
+  kotlin("jvm")
+  alias(libs.plugins.intellij)
+}
+
+repositories {
+  mavenCentral()
+  intellijPlatform { defaultRepositories() }
+}
+
+dependencies { intellijPlatform { create("IC", libs.versions.intellij.get()) } }
+
+spotless {
+  kotlin {
+    ktfmt(libs.versions.ktfmt.get()).googleStyle()
+    target("src/*/kotlin/**/*.kt")
+    licenseHeaderFile(rootProject.file("buildSrc/src/main/resources/license-header.star-block.txt"))
+  }
+}