Patterns: allow custom previews to be supplied

[johngodley]

Patterns are available in the block inserter, and are shown as a preview of the pattern:

For some patterns this preview may be too bulky, or noisy, to give a good experience. Here's a big pattern that takes up too much space, and a small pattern that is hard to see (ignoring that Github shows it zoomed):

A nice enhancement would be if a pattern could support a custom preview or example, much like blocks do. This would also allow for more abstract representations of patterns that may not be easily identifiable.

As the pattern list uses the BlockPreview component then supplying it with a different set of HTML data should be possible.