Block Editor: Add the initial set of block patterns.

Included patterns:

text-two-columns
two-buttons
two-images
text-two-columns-with-images
text-three-columns-buttons
large-header
large-header-paragraph
three-buttons
quote
testimonials

Props mcsf, nrqsnchz.
See #50550.

Built from https://develop.svn.wordpress.org/trunk@48334


git-svn-id: https://core.svn.wordpress.org/trunk@48103 1a063a9b-81f0-0310-95a4-ce76da25c4cd