Port chocolatey install actions to squirrel events

The new squirrel installer installs to `%UserProfile%\AppData\Local\atom` and currently only handles creating start menu and desktop shortcuts.

This PR adds the remaining items that were previously managed by the chocolatey install script.

* Adds an **Open with Atom** file context menu.
* Adds an **Open with Atom** folder context menu.
* Adds `%UserProfile%\AppData\Local\atom\bin` to the `Path` environment variable.
  * Uses `setx` to set the `Path` in the registry.
* Adds a command shim from `%UserProfile%\AppData\Local\atom\bin\atom.cmd` to the latest installed `atom.exe` version since `atom.exe` changes paths on each update.
* Adds a command shim from `%UserProfile%\AppData\Local\atom\bin\apm.cmd` to the latest installed `apm.cmd` version since `apm.cmd` changes paths on each update.


Once Atom ships with this and any bumps are smoothed out, this will be extracted into Atom Shell and added as helper on the `auto-updater` module.

Closes #4180
Everything you possibly can, you should give absolute paths for, so that some dummy who installs MyCoolSetX or something doesn't ruin your day - setx is always at `%SystemRoot%\System32` for example