Add a Windows backend

[kattni]

Adds a toga_winforms backend, so CodeEditor runs on Windows with the same features as the Cocoa, iOS, and Android backends.

• The gutter is a panel docked beside the RichTextBox inside a bordered frame, painted from RichEdit's own line positions (EM_CHARFROMPOS and EM_POSFROMCHAR), so wrapped lines and the trailing empty line need no special cases.
• Highlights are painted through the text box's selection, with undo recording and screen updates suspended through the Text Object Model (ITextDocument.Undo(tomSuspend) and Freeze, reached with ctypes), so Ctrl+Z still undoes typing and the paint does not flicker.
• URL auto-detection and RichEdit's paragraph shortcuts are off, Tab inserts a tab, and Ctrl+V and Shift+Insert paste plain text.
• Entry point, coverage exclusion, README notes, and a Windows section in the example app. No interface changes and no new unit tests: the module cannot import off Windows.

Checked on a Windows 11 ARM64 VM with Python 3.13 and the .NET 10 Desktop Runtime: highlighting, gutter alignment including wrapped and trailing lines, undo, keyboard scrolling, paste, the toolbar controls, and a few-thousand-line buffer.

Known limitations, also in the README: theme colors do not follow the system appearance, text dragged from other apps is not accepted, and the whole buffer is repainted on each debounced pass.

The change note follows in a second commit, named after this PR's number.

PR Checklist:

• I will abide by the BeeWare Code of Conduct
• I have read and have followed the CONTRIBUTING.md file
• This PR was generated or assisted using an AI tool
  Assisted-by: Claude Fable 5.1

[freakboy3742]

A couple of minor cleanups, simplifying the initial widget setup, and some ReST documentation cleanup; but otherwise, this looks great - Thanks!