## Summary
This refreshes the venv activation scripts from upstream `virtualenv`
project.
This was largely triggered by a problem in the activate.nu script (for
nushell):
- #14888 
- #14914 
- #14917 

I was careful to respect the git history going back to #3376
(the last time this was done).
Actually I looked at the complete history from back when this
`uv-virtualenv` crate was named after a Pokémon (⁉️), but I found
nothing (about activation scripts) from back then that hasn't been
overwritten since.

### Some post-processing was involved

- Retained license info at top of scripts
- Retained template vars (eg `{{ BIN_PATH }}`) to assure current support
toward relocatable venv
- Retained deviation from upstream in #5640. This seems to
be the only deviation that isn't in sync with upstream.

### Notable changes from upstream

- (omitted due to undesirable complexity) pypa/virtualenv#2928 and its
follow-up pypa/virtualenv#2940
- pypa/virtualenv#2910 (what prompted #14917 from
#14888)

## Test Plan

There was a request in #14917 to add unit tests to detect breakage or
errors.
I have added a CI job that runs the nushell activation script.
But I think it is better to have the CI test all/most supported shells.
See also #15294 

I have tested this locally using

- [x] nushell (v0.106.1)
- [x] cmd.exe (Microsoft Windows [Version 10.0.26100.4946])
- [x] bash in WSL (GNU bash, version 5.1.16(1)-release
(x86_64-pc-linux-gnu))
- [x] pwsh (PSVersion 5.1.26100.4768)