Restructure/extract Library components

[lyzadanger]

The single module in src/pattern-library/components/Library, containing all of the pattern-library-related components, is getting far too large, but I've avoided making sweeping changes because I don't want to create distracting diffs.

This module should be broken down into multiple component modules. Library could still serve as an entry point that exports what it does currently, such that the API won't change.