REST API: Allow queries other than the main query to be `is_home`.

Ensure that REST API calls can match the main query of the page. 

See #34373.

Fixes #50213.

Props: dlh, TimothyBlynJacobs, boonebgorges, joemcgill, whyisjake. 


Built from https://develop.svn.wordpress.org/trunk@48053


git-svn-id: https://core.svn.wordpress.org/trunk@47820 1a063a9b-81f0-0310-95a4-ce76da25c4cd