Windows is installed through WSL; the native Windows build is withdrawn (from dev a9756dac74c6)

Klammertext on Windows is the Linux build inside WSL 2, or the prebuilt
containers through Docker Desktop. The native build from source (MSYS2) is
no longer offered:

- doc/install/windows_source_install.md and the bundled shell under win/
  are removed.
- doc/install/windows_wsl_install.md is the Windows page. It says there is
  no native build, and its editor section says what holds for each editor:
  Emacs, Vim and Visual Studio Code (through its WSL extension) as on
  Linux; Sublime Text and Notepad++ on the Windows side, with their
  self-contained packages, on files opened through \\wsl.localhost.
- The README lists six install guides; the Notepad++ and VS Code READMEs
  no longer refer to the MSYS2 shell.

No change to the language, the Standard Klammer Set or the commands.

doc/edit/vscode/README.md

@@ -33,9 +33,10 @@ language's structure, everywhere.
 ## Install
 
 Package the extension as a `.vsix` and install it (the builder needs only
-bash and Python — on Windows, run it in the MSYS2 shell the Klammertext
-build uses; the editing zip from the Klammertext website ships a prebuilt
-`klammertext.vsix`, so there the first step is already done):
+bash and Python — on Windows, run it at the Linux prompt of WSL, where
+Klammertext is installed; the editing zip from the Klammertext website
+ships a prebuilt `klammertext.vsix`, so there the first step is already
+done):
 
     ./make_vsix.sh
     code --install-extension klammertext.vsix