Windows: product names, the editors, WSL; the Linux page's image step

Assembled from development commit c3c40f6c8fca. Since 830b193: on Windows the
products carry Windows names (ktext.exe, klammertext.dll, document.dll,
reflect.dll) and the console's output code page is UTF-8 for a run; the
editor tooling works on Windows (VS Code, Sublime Text, Vim, with the
interpreter's name chosen by platform) and Notepad++ joins it
(doc/edit/notepadpp: a User Defined Language and two Run-menu commands);
a sixth install page, doc/install/windows_wsl_install.md, for Linux
inside WSL; the Windows page opens with the three routes and gains its
editor section, the PowerShell quoting and Path notes; the Linux page
installs OpenImageIO from the python3-openimageio package and verifies
with kdiag --machine; the README lists all six install pages. Linux and
macOS behaviour is unchanged.

mac/file.h

@@ -25,9 +25,17 @@ constexpr char path_list_separator = ';';
 #else
 constexpr char path_list_separator = ':';
 #endif
-// The user's home directory as the system names it: HOME, and on Windows
-// USERPROFILE when HOME is unset (a plain PowerShell or cmd sets no HOME;
-// an MSYS2 shell does). Empty when neither is set.
+// The suffix of a shared library loaded at run time (the library a
+// "@eval :cpp" names): ".dll" on Windows, ".so" everywhere else -- the same
+// choice makefile.env's DSO makes when it builds one.
+#ifdef _WIN32
+constexpr const char* shared_library_suffix = ".dll";
+#else
+constexpr const char* shared_library_suffix = ".so";
+#endif
+// The user's home directory as the system names it: HOME; on Windows
+// USERPROFILE first, so the directory is the same from PowerShell (no HOME)
+// and from an MSYS2 shell (which may set HOME to its own /home/<user>).
 std::string home_directory();
 // The first PROGRAM on PATH (split on path_list_separator; on Windows the
 // name also tried with ".exe"), or "" when none: what "which" answers, without