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.

action.yml

@@ -11,38 +11,9 @@ inputs:
     default: 1
 
 runs:
-  using: composite
-  steps:
-    - run: |
-        New-Variable os -Value ('${{ runner.os }}') -Option Constant
-
-        New-Variable windows_host -Value ($os -eq 'Windows') -Option Constant
-
-        New-Variable dirs -Value ('${{ inputs.dirs }}') -Option Constant
-        New-Variable default -Value ('${{ inputs.default }}' -eq '1') -Option Constant
-
-        if ($windows_host) {
-          $sep = [IO.Path]::PathSeparator
-
-          $new_path = $dirs.Split($sep, [System.StringSplitOptions]::RemoveEmptyEntries)
-
-          if ($default) {
-              # This seems to be the default on new installations.
-              # Also, MSYS2 does this.
-              $new_path += @(
-                  'C:\Windows\system32',
-                  'C:\Windows',
-                  'C:\Windows\System32\Wbem',
-                  'C:\Windows\System32\WindowsPowerShell\v1.0\'
-              )
-          }
-
-          $new_path = $new_path -join $sep
-          echo "PATH=$new_path" >> $env:GITHUB_ENV
-        } else {
-          echo "::warning ::Not going to clean up PATH variable on $os"
-        }
-      shell: pwsh
+  using: node12
+  main: clean.js
+  post: restore.js
 
 branding:
   icon: star