Add UI for user to input the media queries for their main content width

[westonruter]

Given that the viewport width is often not the same content width (given sidebars, margins, etc), the calculated heights for the embeds may not be accurate for the media queries. While the generated output includes container queries, those aren't supported by any stable browser yet. Therefore, to ensure that the computed media queries are accurate there needs to be a way for the user to indicate the widths of the main content area (in which the embed will appear) at the various viewport sizes. Ideally this could be done automatically when the tool is used in the context of a site (e.g. as a WordPress plugin).