Try to add a min-width to embeds. (#13590)

* Try to add a min-width to embeds.

Embeds are increasingly responsive (as they should be). This branch is very much a "try" branch, in that it simply zeroes out the minimum width of embeds to make the Amazon book embed be responsive. This allows us to create a nice book recommendation grid like this.

However, Amazon themselves include a "Share" modal inside, which is 320px wide and is `opacity: 0;` (until invoked) instead of `display: none;`. So even though the embed itself is smart enough to not even show the Share button when there isn't room, that zero opacity share sheet still causes a horizontal scrollbar, which due to the sandboxing of iframes, we cannot fix.

So this PR is more of a discussion point: what can we do with these embeds? I'm not sure we can do that much. Notably Instagram widgets can't be smaller than 326px, so we can't do this in a blanket way either, or it'll just mess up other embeds.

What are your thoughts?

* Make min-width not affect placeholders.

* Change to suggested fix.

packages/block-library/src/embed/editor.scss

@@ -9,6 +9,11 @@
 	// Instagram widgets have a min-width of 326px, so go a bit beyond that.
 	@include break-small() {
 		min-width: 360px;
+
+		// The placeholder should not have this min-width.
+		&.components-placeholder {
+			min-width: 0;
+		}
 	}
 
 	&.is-loading {