Contact Form Block: allow same-line labels

### What

Right now our form block puts the field label and the field itself on separate lines, like this:


This user would like the option to let the labels show up next to the fields, like this:

### Why

Just a design preference; passing along [a request from this thread.](https://wordpress.com/forums/topic/single-line-contact-form/)

### How

There are lots of ways we could do this with float or flex or whatever, but for the front end, a toggle on the form block settings would work.

Start by locating the Contact Form Block’s settings and front-end rendering entry points. Confirm how the existing stacked label layout is produced, then add the optional same-line presentation and verify both toggle states in the block preview and rendered form.

Written by the indexing model from the issue text.