Fix text wrapping in Firefox. (WordPress#7472)

* Fix text wrapping in Firefox.

This PR, maybe, fixes WordPress#6049. CC: @SuperGeniusZeb.

It is a one line code change, but this change is put into a highly unscoped and generic location, so I would appreciate both lots of testing and sanity checking that there are no adverse side-effects to doing this.

For reference, this is what we're using: https://developer.mozilla.org/en-US/docs/Web/CSS/overflow-wrap
We could also have used https://developer.mozilla.org/en-US/docs/Web/CSS/word-break

* Scope to block list.

* docs: Tweak comment

editor/components/block-list/style.scss

@@ -136,6 +136,9 @@
 	padding-left: $block-padding;
 	padding-right: $block-padding;
 
+	// Break long strings of text without spaces so they don't overflow the block.
+	overflow-wrap: break-word;
+
 	@include break-small() {
 		// The block mover needs to stay inside the block to allow clicks when hovering the block
 		padding-left: $block-padding + $block-side-ui-padding - $border-width;