bug: issues on VitaminTextField for UIKit

**Describe the bug**

After the review of the `VitaminTextField` on UIKit, we identified some points to fix:
- [ ] The helper text is too close to the bottom of the text field. It should be at 2px (like the counter).
- [ ] The placeholder in dark mode is too visible (content tertiary).
- [ ] When the text field is filled, the text inside should be black (content primary).
- [ ] The label on the `filled` style should be grey (content tertiary).
- [ ] The text field's border in the `active`, `error`, and `success` states should have a thick of 2px.
- [ ] The text field's border in the `disable` state is more visible than when no `disable` > there must be a mistake.
- [ ] The icon for the `filled` style seems to be too high. It should at 4px of the text field.

**Steps to reproduce**

You can see it in the showcase.