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`

crates/uv-python/src/environment.rs

@@ -162,6 +162,11 @@ impl PythonEnvironment {
         Ok(PyVenvConfiguration::parse(self.0.root.join("pyvenv.cfg"))?)
     }
 
+    /// Returns `true` if the environment is "relocatable".
+    pub fn relocatable(&self) -> bool {
+        self.cfg().is_ok_and(|cfg| cfg.is_relocatable())
+    }
+
     /// Returns the location of the Python executable.
     pub fn python_executable(&self) -> &Path {
         self.0.interpreter.sys_executable()