Add elvish activation script

[balthild]

Summary

Supports activation script for elvish

Test Plan

I filled the template variables manually and tried in a project. It works well.

D:\Projects\Python\project-name on  main [?] is 📦 v0.1.0 via 🐍 v3.13.1 
❯ python -V
Python 3.13.1

D:\Projects\Python\project-name on  main [?] is 📦 v0.1.0 via 🐍 v3.13.1 
❯ source .venv\Scripts\activate.elv

D:\Projects\Python\project-name on  main [?] is 📦 v0.1.0 via 🐍 v3.12.8 (project-name) 
❯ python -V
Python 3.12.8

D:\Projects\Python\project-name on  main [?] is 📦 v0.1.0 via 🐍 v3.12.8 (project-name) 
❯ deactivate

D:\Projects\Python\project-name on  main [?] is 📦 v0.1.0 via 🐍 v3.13.1 
❯ python -V
Python 3.13.1

[zanieb]

We'll need to decide if we want to maintain support for additional shells.