Add isCached API to detect @cached decorator usage

- Add computed.isCached() API to detect properties using @cached decorator
- Update EMBER_INSPECTOR_API_ANALYSIS.md with isCached documentation
- Implement stub in ember-inspector-api.js with heuristic detection
- @cached is from @glimmer/tracking and memoizes getter results

This provides a public API to identify cached properties without relying on internal implementation details.

Author: patricklx

.bob/notes/pending-notes.txt

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