fix(computedWithControl): add vue2 compatibility

[vaakian]

Description

Since a ref in vue2 can't be extensible, but we could return the trigger separately. It's off by default for not breaking previous usage, set the third parameter as true to turn it on.

It's being verbose with the third parameter, maybe we could turn it on by default in the next major version.

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[antfu]

Thank you but returning an array violates VueUse's convention. For users who absolutely need Vue2, I would suggest implementing it in their own codebase directly. Thanks for your contribution.