Fix #6008, Add merge strategy for provide option

[znck]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
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• No

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• New/updated tests are included

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[posva]

This is actually not enough because provide can also be a function that returns an object.

I think we should make data strategy reusable and use it for provide
Other plugins like vuefire will also benefit from this 🙂

[znck]

Totally missed that. 😝 I'd add a failing test first.

[znck]

Hey @posva! I guess I didn't miss anything this time.