Editor: Make some hardcoded strings in block patterns translatable.

Props ocean90.
Reviewed by youknowriad, SergeyBiryukov.
Merges [48726] to the 5.5 branch.
Fixes #50842.
Built from https://develop.svn.wordpress.org/branches/5.5@48727


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48489 1a063a9b-81f0-0310-95a4-ce76da25c4cd