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.

settings.gradle.kts

@@ -45,6 +45,8 @@ include("pkl-formatter")
 
 include("pkl-gradle")
 
+include("pkl-internal-intellij-plugin")
+
 include("pkl-parser")
 
 include("pkl-server")