How do I use path the to my VS Code extension's directory in a contributed setting's default value?

I am developing a VS Code extension for Vale style linting. It includes a styles sub-directory with files for individual style rules. Inside the extension, I'm overriding a config option from another extension in its package.json file:

"contributes": {
    "configurationDefaults": {
        "vale.valeCLI.config": ".vale.ini"
    }
 }

The .vale.ini file is included in the root directory of the extension itself. Is there a variable that points to the root directory of the current extension? Or a way derive it from an extension ID? The above code does not work for me...