HTML specifies that a single newline is ignored at the start of a `TEXTAREA`. If `::set_modifiable_text()` is called with a leading newline, ensure it is preserved in the resulting HTML. Developed in https://github.com/WordPress/wordpress-develop/pull/11062. Props jonsurrell, dmsnell. See #64609. Built from https://develop.svn.wordpress.org/trunk@61754 git-svn-id: http://core.svn.wordpress.org/trunk@61060 1a063a9b-81f0-0310-95a4-ce76da25c4cd