Limit pattern shuffling to theme and user patterns only

[richtabor]

While exploring shuffling, we're making strides towards shuffling to relevant patterns, including #60088 and #60074.

Next, I'd like to try shuffling only using the theme and user patterns, not core patterns.

I do think that we can open it back up to core patterns eventually, when they're more suitable for the theme to style. But for now, omitting those will help solidify shuffling with less variables.