Update upgrade CLI to handle containers config correctly

[joulev]

Right now, if you have

{
  containers: {
    foo: "1rem",
  },
}

the generated CSS will be

@theme {
  --containers-*: initial;
  --containers-foo: 1rem;
}

which is incorrect – it should be container not containers. Yes I spent a while thinking the Tailwind IntelliSense extension is not working, but in fact the extension is working fine and it's my CSS file that is invalid.

[philipp-spiess]

Thanks for spotting this!