fix(VTextField): add padding-top custom variable to details

[jeffpohlmeyer]

fixed #12660
fixes #12401

This is a small update to the bug I just recently submitted allowing for explicitly setting the padding-top of .v-text-field__details as error messages get cut off when using some fonts. One line was added to _variables.scss and one line to VTextField.sass.

Description

Resolves #12660 / #12401 by simply adding padding-top custom variable to VTextField

Motivation and Context

Error message in VTextField is cut off on top if using certain fonts like Nunito

How Has This Been Tested?

Has not been tested as this does not change anything because default padding-top on .v-text-field__details was already 0. Now it is just explicitly being set.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• 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:

• The PR title is no longer than 64 characters.
• 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).
• My code follows the code style of this project.
• I've added relevant changes to the documentation (applies to new features and breaking changes in core library)

[MajesticPotatoe]

👍
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