Editor: Disable `wp_keep_scroll_position` in IE11 since buggy; fix `matches`...

Editor: Disable `wp_keep_scroll_position` in IE11 since buggy; fix `matches` polyfill conflict with ME.js by doing runtime feature detection in context window.

Props westonruter, SergeyBiryukov, Clorith for testing.
See #41962, #42029.
Fixes #42553 for trunk.

Built from https://develop.svn.wordpress.org/trunk@42191


git-svn-id: https://core.svn.wordpress.org/trunk@42021 1a063a9b-81f0-0310-95a4-ce76da25c4cd