Block Editor: Rename reusable blocks private APIs to user patterns - #85175

What?

See #57530

Renames the "reusable blocks" private APIs in @wordpress/block-editor (and the matching code in @wordpress/editor) to "user patterns". Public APIs are unchanged.

Why?

Reusable blocks were renamed to patterns in the UI earlier, but much of the code still uses the old name, which makes it harder to follow. #57530 tracks this debt and notes that private APIs can be renamed freely. This PR does that for the private APIs that feed user patterns into the block editor, as a first, low-risk step.

The new name is "user patterns" rather than "synced patterns" because these APIs return every wp_block post, unsynced patterns included. It also matches names the code already uses: the UserPattern type, mapUserPattern(), INSERTER_PATTERN_TYPES.user and userPatternCategoriesSelectKey.

Testing Instructions

1. Create two patterns from the block toolbar's Options menu: one with Synced on and one with it off.
2. Open a new post and open the inserter's Patterns tab. Choose My patterns and check that both patterns are listed.
3. Insert the synced pattern and check that it renders and can be edited.
4. Search the block inserter for the synced pattern by name and insert it from there.
5. Repeat steps 2–4 in the Site Editor, and in the extensible site editor with the gutenberg-extensible-site-editor experiment enabled.

One thing to note: @wordpress/editor reads the settings key from @wordpress/block-editor's private APIs. If someone installs mismatched npm versions of the two packages, user patterns would not show in the inserter. The plugin and Core always ship the two packages together, and private APIs are not meant to work across mismatched versions.

Screenshots or screencast

N/A, no visual changes.