fixes variable references not using reference of current theme

Why does this PR exist?

Closes #2798

Previously when creating variables and different theme groups were affecting the same token structure (e.g. to avoid the figma mode limit), sometimes users would end up with the wrong references.

What does this pull request do?

Adjust the logic around mergeVariableReferences where we now use references from only the selected sets, not from all created themes. This was a mistake in the first place.

Testing this change

Create the following sets:

{
  "global": {
    "red": {
      "$value": "#ff0000",
      "$type": "color"
    }
  },
  "globalB": {
    "red": {
      "$value": "#f80",
      "$type": "color"
    }
  },
  "semanticA": {
    "warning": {
      "$value": "{red}",
      "$type": "color"
    }
  },
  "semanticB": {
    "warning": {
      "$value": "{red}",
      "$type": "color"
    }
  }

and then create theme groups like this:

And create variables - one theme group at a time (first A, then B). The references will then pick the right ones. If you create all themes at once, it will still have the wrong reference, as it will just use the red reference from the first theme.

Additional Notes (if any)

I don't think this is the final solution. I'd love for us to isolate the variable creation and update logic and possibly extract that to its own package. As in we pass in a list of tokens and a theme configuration, and the package handles everything when it comes to creation and updates of variables. Right now variable creation and updating that one is quite scattered and really hard to follow (not having worked on the original solution I found it hard to grasp how everything tied together)

tokens-studio / figma-plugin