Accessibility: Insert Link modal: Improve keyboard interaction.

Avoids change the selected link when using the Shift + Up/Down arrow keys
to select text in the form fields.

Props afercia.

Merges [42807] to the 5.0 branch.
Fixes #43253.

git-svn-id: https://develop.svn.wordpress.org/branches/5.0@44405 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/js/wplink.js

@@ -559,7 +559,7 @@ var wpLink;
 			}
 
 			// Up Arrow and Down Arrow keys.
-			if ( 38 !== event.keyCode && 40 !== event.keyCode ) {
+			if ( event.shiftKey || ( 38 !== event.keyCode && 40 !== event.keyCode ) ) {
 				return;
 			}