Add support for Stew

[Jelloeater]

I want to suggest a new step

• Which tool is this about? Where is its repository?

• Stew

• https://github.com/marwanhawari/stew

• Which operating systems are supported by this tool?

• Windows, Linux and Mac

• What should Topgrade do to figure out if the tool needs to be invoked?

• Should run upgrade process on system AND user bins

• Which exact commands should Topgrade run?
  stew upgrade --all

• Does it have a --dry-run option? i.e., print what should be done and exit

• Don't think it has one

• Does it need the user to confirm the execution? And does it provide a --yes
  option to skip this step?

• No, it runs unattended, but may prompt if it the bin filename on GitHub changes.

I want to suggest some general feature

Topgrade should...

More information

[SteveLauC]

This has already been implemented in #422, try:

$ topgrade --only stew