Smaller field width option for modal forms

[julianbonilla]

When creating forms in a modal, it would be nice to have a class with a smaller field width. We don't have a way to display multiple fields in a row for modals.

rx-field elements consume between 250px and 1/3 of the width. We have the controlled-width class, which uses @rxForm-field-max-width: 250px. That's too wide for a modal context. I think 100px would allow for up to three fields in a row.

Here's an example override using flex: 1 0 100px; for rx-field: