Output path placeholders don't expose ValueRenderer.extension

[lilyball]

The --output-path flag supports placeholders for generating the output filename from inputs. Unfortunately there is no placeholder that will provide the file extension. %{outputFormat} exists but it corresponds to the --format flag rather than the actual ValueRenderer subclass used, and it's the name of the format rather than the extension (which is the same for some formats, but not for others).

I'd like to see two things. First, a new placeholder %{outputExtension} that evaluates to if (output.renderer.extension != null) ".\(output.renderer.extension)" else "". The inclusion of the leading period here is because ValueRenderer.extension is optional. This way I can write something like pkl eval -o 'config%{outputExtension}' config.pkl and get the appropriate extension no matter how it's rendering. The second is a shorthand for %{moduleName}%{outputExtension}. This could be another placeholder, but for ease of use I'd prefer a separate flag -O, --output-dir that takes a directory (supporting placeholders). So with this, -O dir would be equivalent to saying -o dir/%{moduleName}%{outputExtension}.

Given that the Pkl Style Guide recommends that Pkl files representing static config files be named to match the target filename, this means the -O flag would be the simplest way to render static config files.