fix(VComboBox): add "allow-overflow" prop

[Z3rio]

Description

Before this change, adding "allow-overflow" as a prop to v-combobox would do nothing. But with this change, setting allow-overflow to true will allow overflow (scrolling), this is also the default if this prop isnt passed through. Upon setting allow overflow to false, scrolling is turned off.

Resolves issue: #16008

Motivation and Context

This would be needed because alot of people would like to display the combo list in its entirety instead of requiring the user to scroll through the list to see everything.

How Has This Been Tested?

I have tested this with both the prop set to false, true and also without the prop. I also ran the tests included in vuetify.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Improvement/refactoring (non-breaking change that doesn't add any features but makes things better)

Checklist:

• [x] The PR title is no longer than 64 characters.
• [x] The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and backwards compatible changes and next for non-backwards compatible changes).
• [x] My code follows the code style of this project.
• [x] I've added relevant changes to the documentation (applies to new features and breaking changes in core library)