feat(nu): transient prompt

[JanDeDobbeleer]

Prerequisites

• I have read and understood the contributing guide.
• The commit message follows the conventional commits guidelines.
• Tests for the changes have been added (for bug fixes / features).
• Docs have been added/updated (for bug fixes / features).

Description

🤖 Generated by Copilot at b007609

Refactor and update the omp.nu script to enable the transient prompt feature for Nushell. Update the documentation to reflect the new feature and the minimum version requirement of Nushell v0.86.0.

How

🤖 Generated by Copilot at b007609

• Refactor omp.nu script to define helper functions and enable transient prompt feature for Nushell (link)
• Update documentation to include Nushell as a supported shell for the transient prompt feature and require Nushell v0.86.0 or higher for Oh My Posh (link, link, link)