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.

doc/edit/vscode/make_vsix.sh

@@ -9,7 +9,7 @@
 #     code --install-extension klammertext.vsix
 #
 # A .vsix is a zip holding a two-file manifest plus the extension payload
-# under extension/.  This script builds it with bash + python3 only — no
+# under extension/.  This script builds it with bash + Python only — no
 # npm, no vsce — matching the extension's no-dependency design.  The shared
 # core and language server are vendored into the payload: from this folder
 # if present (the editing-zip layout), else from ../shared (the repository
@@ -18,6 +18,12 @@
 set -e
 cd "$(dirname "$0")"
 
+# The interpreter: $PYTHON if the environment names one (env/runtime.env
+# does), else python3, else python -- the only name python.org installs on
+# Windows, where this runs in the MSYS2 shell.
+PYTHON="${PYTHON:-$(command -v python3 || command -v python)}"
+[ -n "$PYTHON" ] || { echo "make_vsix.sh: no python3 or python on PATH" >&2; exit 1; }
+
 OUT="$PWD/klammertext.vsix"
 STAGE="$(mktemp -d)"
 trap 'rm -rf "$STAGE"' EXIT
@@ -36,7 +42,7 @@ for f in klammertext_edit.py klammertext_ls.py; do
     fi
 done
 
-VERSION=$(python3 -c "import json; print(json.load(open('package.json'))['version'])")
+VERSION=$("$PYTHON" -c "import json; print(json.load(open('package.json'))['version'])")
 
 cat > "$STAGE/[Content_Types].xml" <<'EOF'
 <?xml version="1.0" encoding="utf-8"?>
@@ -75,7 +81,7 @@ cat > "$STAGE/extension.vsixmanifest" <<EOF
 </PackageManifest>
 EOF
 
-python3 - "$STAGE" "$OUT" <<'EOF'
+"$PYTHON" - "$STAGE" "$OUT" <<'EOF'
 import os, sys, zipfile
 stage, out = sys.argv[1], sys.argv[2]
 with zipfile.ZipFile(out, 'w', zipfile.ZIP_DEFLATED) as z: