make it into a JavaScript action

This is a huge step back IMO, but I needed to be able to restore the
original PATH back as a "post" step.  Currently, composite actions don't
support post-actions, but JavaScript ones do.  I needed this due to a
bug: actions/cache wouldn't find Git's tar on windows-2016 (the one in
System32 would get used on windows-2019) if the PATH was cleaned up.

package.json

@@ -0,0 +1,22 @@
+{
+  "name": "cleanup-path",
+  "version": "2.0.0",
+  "description": "Clean up PATH on Windows workers",
+  "main": "clean.js",
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1"
+  },
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/egor-tensin/cleanup-path.git"
+  },
+  "author": "Egor Tensin <Egor.Tensin@gmail.com>",
+  "license": "MIT",
+  "bugs": {
+    "url": "https://github.com/egor-tensin/cleanup-path/issues"
+  },
+  "homepage": "https://github.com/egor-tensin/cleanup-path",
+  "dependencies": {
+    "@actions/core": "^1.2.6"
+  }
+}