Adds drop handler and MSI registration.

[zooba]

Fixes #217

[zooba]

Only a draft right now, because it doesn't load properly when installed via a test MSI (works fine if I hack in the registration). I need to do more testing, but can't do it right now.

Isn't going to work at all for the MSIX, so this will also need a troublshooting entry over on the docs.

[zooba]

It works fine on a clean machine, so let's take it.

[Copilot]

Pull Request Overview

This PR adds drag-and-drop support for Python files by implementing a new COM drop handler (DragDropSupport class) and registering it in both MSI and MSIX/AppX packages. When files are dragged onto a Python script, the drop handler will execute the script with the dropped files as arguments.

Key Changes:

• Implemented DragDropSupport COM class with IDropTarget and IPersistFile interfaces
• Added drop handler registration in MSI for .py and .pyw file types
• Added context menu registration for .pyw files in AppX manifest
• Added test coverage for argument parsing and drop description functionality

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
src/pyshellext/shellext.cpp	Implements DragDropSupport COM class for handling drag-and-drop operations, including argument parsing from dropped files and execution logic
src/pyshellext/shellext.h	Adds function declarations for new drop-related helper functions
src/pyshellext/shellext_test.cpp	Adds Python test wrapper functions for the new drop argument parsing and description functions
tests/test_shellext.py	Adds unit tests for argument parsing (wide and ASCII) and drop description extraction
src/pymanager/msi.wxs	Registers the DragDropSupport COM class and associates it as a drop handler for Python.File and Python.NoConFile ProgIds
src/pymanager/appxmanifest.xml	Adds context menu verb registration for .pyw files using the EditInIdle command
_msbuild.py	Adds pathcch.lib dependency required for path manipulation in the drop handler
_msbuild_test.py	Adds pathcch.lib dependency and exports new test functions for the drop handler

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