For 6.3 Beta

As we are taking the opportunity to unify some concepts and streamline how users work with patterns, reusable blocks, and template parts, we ended up using the generic label Library. I think this is suboptimal for a few reasons, so I propose calling it just Patterns.

Rationale

• Discovery: this is an opportunity to make patterns front and center as we are introducing the ability to save custom patterns. They should be front and center in the "Design" tab.
• Clarity: library can be obscure and overlaps with other terminology (like Media Library). We didn't get to add font library management in this round, but it would have made things more confusing (would a user expect to see their font library under "Styles" or an item called "Library"?). Calling it patterns removes that ambiguity.
• Presence: patterns is a unique name that has been established in the WP branding parlance for a bit and deserves more clear placement.