fix: Improve accessibility of toolbox search

[gonfunko]

The basics

• I validated my changes

The details

Fixes #2613

Proposed Changes

This PR fixes and improves a number of keyboard and screenreader issues with the toolbox search plugin:

• Resolves some situations where the focus manager could get into a bad state
• Clarifies the placeholder text so the purpose of the field is more evident to screenreader users
• Improves keyboard navigation: left and right arrows can navigate character by character, moving focus to the matching blocks once the end of the search string is reached; up and down move to the beginning/end of the search string and then to the previous/next toolbox category
• Fixes a bug that prevented clicking the clear button in the field in Safari
• Announces the number of matching blocks to screenreaders so that users know when/if they should navigate to the results list
• Removes some now-dead code