Support for Pkl

[sgammon]

Feature description

It would be cool to support Apple's Pkl language. Elide could support this as a configuration and static asset language. Pkl structures can render to static configuration (JSON, YAML, plists, and so on), and the language implementation is based on Truffle. Elide's fork of Pkl supports the latest APIs for GraalVM and is already compatible with our Gradle build via a Git submodule and includeBuild(...).