chore: add changeset for composables export feature

- Add changeset documentation for PR #295
- Tracks the composables export feature for release notes

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: Seungwoo321

.changeset/composables-export-patch.md

@@ -0,0 +1,11 @@
+---
+"vue-pivottable": patch
+---
+
+Export all composables for external packages to enable custom renderer development
+
+- Added `export * from './composables'` to main index.ts
+- Resolves import errors for `useProvidePivotData` and `providePivotData` in external packages
+- Enables plotly-renderer and lazy-table-renderer to properly import required functions
+- No circular dependency issues detected after thorough analysis
+- Maintains backward compatibility while improving extensibility