Improve `.is-required` star display on select input

Prerequisites

• I have searched for duplicate or closed issues
• I have validated any HTML to avoid common problems
• I have read the contributing guidelines

Describe the issue

Right now is-required markers are not visible when the label is longer than the select input when one option is selected.

This is due to the fact that the label is absolutely positionned and we cannot use the position: relative like in text input.

Investigation ideas: (no idea if these would work):

• use a child container to support the "position: relative"
• use container queries to set the rightproperty of the marker only when the label spans the whole input (using container query units?)

Reduced test cases

See before

What operating system(s) are you seeing the problem on?

macOS

What browser(s) are you seeing the problem on?

Firefox

What version of Boosted are you using?

v1.3.0

Start by reproducing the .is-required marker issue on a select input with a label longer than the control and one option selected, using the reported Firefox/macOS context. Inspect the select-input styling entry point and compare it with the text-input marker behavior. Done means the required star remains visible without breaking the layout in the reported case.