Fix nunjucks use in document builder by calling mapArgs in non-extension context

[twschiller]

Context

• Document builder's list element calls mapArgs for !defer content: #1976 Document Builder. Lists #2077
• Our CSP for the extension page (correctly) blocks it because nunjucks uses eval under the hood

Implementation Sketch

• Expose a method to run mapArgs in the contentScript API (similar to runEffectPipeline)
• Call that method instead of calling mapArgs directly

Related

• #1976 Document Builder. Lists #2077
• Move template engine and brick evaluation to Chrome sandbox #105 (Ideally we'd run it in the sandbox instead of the host page, long-term we'll likely need to fork nunjucks to remove the unsafe dynamic behavior)
• Composite Renderer bricks should run non-renderer steps in content script #1939