feat: slicers & timeline filters

[productdevbook]

Summary

Add slicer and timeline slicer support for pivot table and table filtering UI.

Technical details

Slicers (xl/slicers/slicer1.xml):

• Visual filter controls for PivotTables and Tables
• <slicer> elements with name, cache ref, caption, columnCount, style

Slicer caches (xl/slicerCaches/slicerCache1.xml):

• Data source and filter state

Timeline slicers (Excel 2013+):

• Date-based filter with granularity (years, quarters, months, days)

Scope

• Read slicer definitions
• Write slicers for Tables
• Write slicers for PivotTables
• Timeline slicers
• Round-trip preservation

References

• MS-XLSX slicers: https://learn.microsoft.com/en-us/openspecs/office_standards/ms-xlsx/bfeb551b-f590-4f68-aa8d-9edd824acefc