fix: install PowerShell completions

[LouisDeconinck]

Summary

Install the bundled PowerShell completion script when an existing PowerShell completions directory is available, while preserving the current hint otherwise.

Checks

• git diff --check
• Manual review for Windows PowerShell 5.1-compatible syntax

Closes #139

[github-actions]

Thanks for your first pull request to Sotto, and welcome.

Two things that trip up newcomers:

• Every commit needs a Signed-off-by line under the Developer Certificate of Origin - PRs with unsigned commits cannot be merged. Sign off with git commit -s; if your branch already has unsigned commits, run git rebase --signoff main and push again.
• Please run cargo fmt --all --check and cargo clippy --workspace --all-targets -- -D warnings before pushing, and add tests for new behaviour (cargo test --workspace runs the suite).

The full guide is in CONTRIBUTING.md - a maintainer will review shortly.