Closes https://github.com/payloadcms/payload/issues/7867
Problem: currently, setting an
```ts
admin: {
width: '30%'
}
```
does not work for fields inside a row or similar (group, array etc.)
Solution: when we render the field, we set a CSS variable
`--field-width` with the value of `admin.width`. This allows us to
calculate the correct width for a field in CSS by doing `flex: 0 1
var(--field-width);`
It also allows us to properly handle `gap` with `flex-wrap: wrap;`
Notes: added playwright tests to ensure widths are correctly rendered
cd734b0f98