feat: Add mounts option to modularEffect

[roberth]

Motivation

Make use of __hci_effect_mounts convenient - mount any path into the effect, as permitted by the agent.

Maintainer checklist

• [x] Documentation (function reference docs, setup guide, option reference docs)
• [x] Has tests (VM test, free account, and/or test instructions)
• [ ] Is the corresponding module up to date?