fix: checkMFPlugin function support array plugins

[2heal1]

Summary

plugins may pass array type like this:

{
      format: 'mf',
      plugins: [
        [
          { name: 'rsbuild:module-federation-enhanced', setup: () => {} },
          { name: 'plugin-foo', setup: () => {} },
        ],
      ],
}

Related Links

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

[codspeed-hq]

CodSpeed Performance Report

Merging #419 will not alter performance

_{Comparing fix/rsbuild-plugins (f813069) with main (dacb1d2)}

Summary

✅ 5 untouched benchmarks