Pkl support

[zml2008]

Pkl is a new configuration language: https://pkl-lang.org/

Does it make sense to support this in Configurate? It's somewhere along the spectrum between config format and scripting language -- almost like the nix language, but it does have a nice java library. Looking at something like hocon, it's not too much more complex

To begin to support this language, we'd want to extend configurate's object model with a few things:

• lazy-/reference-valued nodes
• lazily-evaluated interpolated strings