Texturize: Transform `&` into `&` in tag attributes.

[35709] was overly broad, and stopped transforming `&` characters within tag attributes. So that sites aren't generating invalid HTML, we need to restore this functionality, while continuing to not transform `&` within blocked tags.

Fixes #35008 for trunk.


Built from https://develop.svn.wordpress.org/trunk@36036


git-svn-id: https://core.svn.wordpress.org/trunk@36001 1a063a9b-81f0-0310-95a4-ce76da25c4cd