Revert "Remove unused npx binary"

This reverts commit f019a24.

Git Bash on Windows still uses this file. Removing it broke
the windows MSI builds in Node.js infrastructure. Unless we
absolutely need to remove this I think it is worth keeping
around

PR-URL: npm#1903
Credit: @MylesBorins
Close: npm#1903
Reviewed-by: @isaacs

bin/npx

@@ -0,0 +1,39 @@
+#!/bin/sh
+
+# This is used by the Node.js installer, which expects the cygwin/mingw
+# shell script to already be present in the npm dependency folder.
+
+(set -o igncr) 2>/dev/null && set -o igncr; # cygwin encoding fix
+
+basedir=`dirname "$0"`
+
+case `uname` in
+    *CYGWIN*) basedir=`cygpath -w "$basedir"`;;
+esac
+
+NODE_EXE="$basedir/node.exe"
+if ! [ -x "$NODE_EXE" ]; then
+  NODE_EXE=node
+fi
+
+NPM_CLI_JS="$basedir/node_modules/npm/bin/npm-cli.js"
+NPX_CLI_JS="$basedir/node_modules/npm/bin/npx-cli.js"
+
+case `uname` in
+  *MINGW*)
+    NPM_PREFIX=`"$NODE_EXE" "$NPM_CLI_JS" prefix -g`
+    NPM_PREFIX_NPX_CLI_JS="$NPM_PREFIX/node_modules/npm/bin/npx-cli.js"
+    if [ -f "$NPM_PREFIX_NPX_CLI_JS" ]; then
+      NPX_CLI_JS="$NPM_PREFIX_NPX_CLI_JS"
+    fi
+    ;;
+  *CYGWIN*)
+    NPM_PREFIX=`"$NODE_EXE" "$NPM_CLI_JS" prefix -g`
+    NPM_PREFIX_NPX_CLI_JS="$NPM_PREFIX/node_modules/npm/bin/npx-cli.js"
+    if [ -f "$NPM_PREFIX_NPX_CLI_JS" ]; then
+      NPX_CLI_JS="$NPM_PREFIX_NPX_CLI_JS"
+    fi
+    ;;
+esac
+
+"$NODE_EXE" "$NPX_CLI_JS" "$@"