feat(venv): add relocatable flag (#5515)
## Summary
Adds a `--relocatable` CLI arg to `uv venv`. This flag does two things:
* ensures that the associated activation scripts do not rely on a
hardcoded
absolute path to the virtual environment (to the extent possible; `.csh`
and
`.nu` left as-is)
* persists a `relocatable` flag in `pyvenv.cfg`.
The flag in `pyvenv.cfg` in turn instructs the wheel `Installer` to
create script
entrypoints in a relocatable way (use `exec` trick + `dirname $0` on
POSIX;
use relative path to `python[w].exe` on Windows).
Fixes: #3863
## Test Plan
* Relocatable console scripts covered as additional scenarios in
existing test cases.
* Integration testing of boilerplate generation in `venv`.
* Manual testing of `uv venv` with and without `--relocatable`
