mix mneme.watch: Use previously selected pattern to select default patterns

[zachallaun]

Imagine you're running mix mneme.watch and, in the CLI, you've used K to see the most complex pattern value. You then change the code and the value changes somewhat, but you have to press K again to see the pattern you want.

It would be neat if mneme.watch could remember the last viewed pattern when selecting a default pattern for a particular assertion.