Windows is installed through WSL; the native Windows build is withdrawn (from dev a9756dac74c6)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/make_vsix.sh

@@ -20,7 +20,7 @@ cd "$(dirname "$0")"
 
 # The interpreter: $PYTHON if the environment names one (env/runtime.env
 # does), else python3, else python -- the only name python.org installs on
-# Windows, where this runs in the MSYS2 shell.
+# Windows, should this run in a bash there.
 PYTHON="${PYTHON:-$(command -v python3 || command -v python)}"
 [ -n "$PYTHON" ] || { echo "make_vsix.sh: no python3 or python on PATH" >&2; exit 1; }