Add/combobox loading spinner author part

[adamsilverstein]

What?

Add a loading indicator to the author dropdown. Fix a bug where it was impossible to tell a search was occuring, leating to poor UX especially over slower connections.

Follow up to #68990

Post author part of #68927

Testing instructions

1. Create a large number of editors (see below for command).
2. Load a page in the editor
3. Open DevTools and throttle network connections to a 3G speed to slow down the rest API responses
4. Click the author to edit, then try typing in part of a user name

Expected results:

• A spinner is shown while the search callback executes
• the "No items found' string only shows when the search completes (and no items are found)

Helpful wp-cli commands for testing:

Generate many users to test searching: wp user generate --role=editor --count=100
Delete test users: wp user delete $(wp user list --role=editor --field=ID) --reassign=2

Screencasts

Before changes (trunk)

before2.mp4

Add spinner

(note message "No items found" shows during search callback)

spinner-only.mp4

Final PR

(including removal of "No items found" during load)

after.mp4

[github-actions]

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Co-authored-by: adamsilverstein <[email protected]>

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