[postcss-plugin] Fix importSource object syntax not working properly

[javascripter]

What changed / motivation ?

Using the { from, as } object form in importSources caused a runtime error due to incorrect .includes usage in Previous PR. This fix ensures both string and object forms are handled properly in shouldTransform.

Linked PR/Issues

#1063

Additional Context

In main branch, move to the example app

cd examples/example-nextjs

Using the object syntax throws when running npm run example:dev:

   importSources: [
        {
          // Note:Just enough config to reproduce the error, not actually a proper config for stylex
          from: '@stylexjs/stylex',
          as: 'create',
        },
      ],

Switch to this branch and now it does not throw and renders correctly.

Pre-flight checklist

• I have read the contributing guidelines
  Contribution Guidelines
• Performed a self-review of my code

[necolas]

We need unit tests for this plugin