feat: add UseSharedStrings option for streaming writes (D1)

[AdamDrewsTR]

Summary

Add UseSharedStrings option for streaming writes. When enabled, the stream writer stores unique string values once in xl/sharedStrings.xml and references them by index in cell elements (t="s" with <v>idx</v>), instead of writing inline strings for every cell.

This can significantly reduce output file size when many cells contain duplicate string values.

Implementation

• Add UseSharedStrings bool to Options struct
• Add useSharedStrings, sharedStrings, sharedStringsMap fields to StreamWriter; initialized from Options in NewStreamWriter
• After setCellValFunc produces an inlineStr cell with a simple <t> value, convert it to a shared string reference (t="s")
• getOrAddSharedString: dedup via map, returns shared string index
• writeSharedStrings: on Flush, writes xl/sharedStrings.xml with proper xml:space="preserve" for whitespace-padded strings, adds content type part and workbook relationship if not already present

Usage

f := excelize.NewFile()
f.SetOptions(excelize.Options{UseSharedStrings: true})
sw, err := f.NewStreamWriter("Sheet1")
// ... write rows as usual ...
sw.Flush()
f.SaveAs("output.xlsx")

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.60%. Comparing base (4bebb61) to head (bfedfc5).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2326   +/-   ##
=======================================
  Coverage   99.60%   99.60%           
=======================================
  Files          32       32           
  Lines       26791    26838   +47     
=======================================
+ Hits        26685    26732   +47     
  Misses         55       55           
  Partials       51       51           

Flag	Coverage Δ
unittests	99.60% <100.00%> (+<0.01%)	⬆️

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