fix(bridge): define mixin outside plugin, to avoid memory leak

[danielroe]

🔗 Linked issue

follow up on #3898

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

this defines the meta mixin outside the plugin so we can add the installed flag in bridge...

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[pi0]

Since we are already guarding bridge-compat, is this change necessary?

[danielroe]

The issue is that the mixin object is defined inline, which means the __nuxt_installed flag is set freshly each time, and it can't deduplicate. By moving it out of function, we get the protection applied.

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