`update-shell` should support force pre-pending to the `PATH`

[zanieb]

Summary

Currently, we will exit if the directory is on the PATH at all

uv/crates/uv/src/commands/python/update_shell.rs

Lines 45 to 52 in d525720

	if Shell::contains_path(&executable_directory) {
	writeln!(
	printer.stderr(),
	"Executable directory {} is already in PATH",
	executable_directory.simplified_display().cyan()
	)?;
	return Ok(ExitStatus::Success);
	}

but this does not allow updating the SHELL in the case where the directory is at the end of the path and binaries are shadowed.

Either we should detect that the relevant binary is shadowed and do this automatically, or we should allow some sort of --force option to prepend even if it's present already.

Example

uv python update-shell --force