Increases the frequency of heartbeat API requests from once every 15 seconds to once every 10 seconds.
The purpose of this change is to reduce the length of time before a post becomes unlocked as a user navigates around the WordPress Dashboard and ceases editing a post.
`wp.heartbeat.interval()` has been modified to allow theme and plugin authors to set the heartbeat interval to any value between one second and one hour rather than limiting them to a fixed set of values.
Props azaozz, annezazu, jorbin, kirasong.
Fixes#61960.
Built from https://develop.svn.wordpress.org/trunk@59016
git-svn-id: http://core.svn.wordpress.org/trunk@58412 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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