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: astral-sh#3863