Add a Sass executable wrapper that forwards to the correct exe

This allows users to run `npx sass` when they've just installed
`sass-embedded` and directly run the Dart VM. Due to npm/cmd-shim#152,
there's no way to make this work on Windows CMD/Powershell without
substantially curtailing the performance on other operating systems.

bin/sass

@@ -0,0 +1,62 @@
+#!/bin/bash -e
+
+dir="$(cd "$(dirname "${BASH_SOURCE[0]}")"; pwd)"
+if [ "$(basename "$dir")" == bin ]; then
+    # Global executables are installed in .../bin/, and global libraries
+    # (including their dependencies) in .../lib/node_modules.
+    node_modules="$(dirname "$dir")/lib/node_modules"
+elif [ "$(basename "$dir")" == .bin ]; then
+    # Local dependency executables are installed in ./node_modules/.bin, and
+    # and their libraries in ./node_modules.
+    node_modules="$(dirname "$dir")"
+else
+    # If all else fails, try to find node_modules in a parent directory. We
+    # might execute this, for example, if someone runs the script from source.
+    while [ "$dir" != "/" ]; do
+        dir="$(dirname "$dir")"
+        if [ -d "$dir/node_modules" ]; then
+            node_modules="$dir/node_modules"
+            break
+        fi
+    done
+    echo "Could not find node_modules above ${BASH_SOURCE[0]}!" >&2
+    exit 1
+fi
+
+case "$(uname -s)" in
+    Linux*)
+        if [ ! -z "$ANDROID_ROOT" -a ! -z "$ANDROID_DATA" ]; then
+            os=android
+        else
+            os=linux
+        fi;;
+    Darwin*)
+        os=darwin;;
+    CYGWIN*|MINGW*|MSYS_NT*)
+        os=win32;;
+    *)
+        echo "Unknown operating system $(uname -s)!" >&2
+        exit 1
+esac
+
+musl=
+if [ "$os" == linux ] && grep -q /ld-musl- /bin/bash; then musl=-musl; fi
+
+case "$(uname -m)" in
+    aarch64*) arch=arm64;;
+    arm*) arch=arm;;
+    x86_64) arch=x64;;
+    i386|i486|i586|i686) arch=ia32;;
+    *)
+        echo "Unknown architecture $(uname -m)!" >&2
+        exit 1
+esac
+
+shared="$node_modules/sass-embedded-$os$musl-$arch"
+specific="$node_modules/sass-embedded/node_modules/sass-embedded-$os$musl-$arch"
+if [ -d "$specific" ]; then
+    exec "$specific/dart-sass/sass" "$@"
+else
+    exec "$shared/dart-sass/sass" "$@"
+fi
+

package.json

@@ -23,6 +23,7 @@
   "engines": {
     "node": ">=16.0.0"
   },
+  "bin": {"sass": "bin/sass"},
   "scripts": {
     "init": "ts-node ./tool/init.ts",
     "check": "npm-run-all check:gts check:tsc",