Add PowerShell tab completion support

[HeyItsGilbert]

Summary

This PR adds PowerShell tab completion support to shtab, enabling native PowerShell argument completion for CLI applications built with argparse.

Key features:

• Generates PowerShell completion scripts using Register-ArgumentCompleter -Native
• Supports PowerShell 5.1+ (Windows Desktop) and PowerShell Core 7.x (cross-platform)
• Automatically discovers subcommands, options, choices, and file/directory completions
• Follows the same architecture as existing bash/zsh/tcsh support
• Includes examples and comprehensive documentation

Changes

• shtab/init.py: Added PowerShell completer implementation with:

  • get_powershell_commands() for parser tree extraction
  • complete_powershell() decorated completer function
  • PowerShell script template with state machine for completion dispatch
  • Serialization helpers for PowerShell hashtable generation
  • Integration with existing CHOICE_FUNCTIONS infrastructure

• Documentation & Examples:

  • Updated README.rst and docs/index.md to list PowerShell as supported shell
  • Added PowerShell installation & usage guide in docs/use.md
  • Updated examples/customcomplete.py with PowerShell preamble

• Tests: Extended existing parametrized test suite to include PowerShell

Building & Testing Locally

Prerequisites

pip install -e .

Run tests

# All tests (including PowerShell smoke tests)
pytest tests/test_shtab.py -v

# PowerShell-only tests (requires pwsh)
pytest tests/test_shtab.py -k "powershell" -v

Generate completion script manually

# For shtab itself
shtab --shell=powershell shtab > shtab.ps1

# Or for an example app
python -c "from examples.customcomplete import main; print(main)" | shtab --shell=powershell > custom.ps1

Test completion in PowerShell

# Load the generated script
. ./shtab.ps1

# Test completions (Ctrl+Space or type -<TAB>)
shtab -<TAB>

Enable debug output

Set the SHTAB_DEBUG environment variable to trace completion state:

$env:SHTAB_DEBUG=1
. ./shtab.ps1