Add term to public_bookcase.

[Coehill]

Description, Motivation & Context

Public bookcases are commonly referred to as "little free libraries", so let's include this an a naming option.

Related issues

Closes #1332

Links and data

Relevant OSM Wiki links:

• …

Checklist and Test-Documentation Template

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## Test-Documentation

### Preview links & Sidebar Screenshots

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### Wording

- [ ] American English
- [ ] `name`, `aliases` (if present) use Title Case
- [ ] `terms` (if present) use lower case, sorted A-Z
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