Refactor CSS of label positioning

[sebald]

Description

We allow to position a label on top of an input (default) but themes, like the theme-core can also place the label on the left side. Especially the later one is channeling because all labels must have the same width in a form. This is why we introduced <FormGroup/>. Currently there are some placement issues with the left aligned label we want to adress:

• We need a default label width because otherwhise the label will collapse sometimes
• (We need more control over the placement)

Proposal

• Use CSS grid to make easier to switch between the two layouts, using grid-area (see image above)
• Maybe introduce a labelPosition prop on the FieldGroup?

[sarahgm]

think we can close this with #2911 reasons