Add promptfoo to Tools & Libraries

Author: mldangelo

pages/tools.en.mdx

@@ -54,6 +54,7 @@
 - [Prompter](https://prompter.engineer)
 - [PromptInject](https://github.com/agencyenterprise/PromptInject)
 - [Prompts.ai](https://github.com/sevazhidkov/prompts-ai)
+- [promptfoo](https://github.com/promptfoo/promptfoo)
 - [Promptmetheus](https://promptmetheus.com)
 - [PromptPerfect](https://promptperfect.jina.ai/)
 - [Promptly](https://trypromptly.com/)