fix(types): Fix missing function type declaration for ref attribute

[dafengxiang]

The ref attribute supports incoming strings and functions, but the function type declaration is not provided in the type declaration

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

• [x] Bugfix
• [ ] Feature
• [ ] Code style update
• [ ] Refactor
• [ ] Build-related changes
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• [ ] Yes
• [x] No

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• [x] It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
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• [ ] All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• [ ] New/updated tests are included

If adding a new feature, the PR's description includes:

• [ ] A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

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