This closes #2191, add Rows.IterateColumns to stream cells without []string allocation

[artur-chopikian]

Description

Adds a new Rows.IterateColumns method that streams each non-empty cell of the current row through a user-supplied callback func(colIndex int, cellValue string), instead of
returning a []string like Rows.Columns does.

Internally, the SAX parse loop in Columns is extracted into a private iterate helper that both methods share. rowXMLIterator gains a yield field; when set, rowXMLHandler
calls it per cell and skips the append(appendSpace(...), val) accumulation. When unset, the original code path runs verbatim — Columns keeps its existing behavior and pays only
a single nil check per cell.

Related Issue

Closes: #2191.

Motivation and Context

Rows.Columns allocates and grows a []string for every row, and fills middle gaps between non-empty cells with "". On wide or sparse rows this is a significant amount of work
the caller doesn't actually need — most callers immediately iterate the slice and never retain it. IterateColumns lets callers consume cells directly, eliminating the slice
allocation, the append regrowth chain, and the gap-filling loop, while preserving the existing Columns API and performance.

Usage:

rows, _ := f.Rows("Sheet1")
for rows.Next() {
    err := rows.IterateColumns(func(colIndex int, cellValue string) {
        fmt.Printf("col %d: %s\n", colIndex, cellValue)
    })
    if err != nil { /* ... */ }
}
rows.Close()

How Has This Been Tested

Added TestIterateColumns in rows_test.go covering:

• Parity with Columns: iterates test/Book1.xlsx Sheet2 with both APIs and compares the resulting rows after trimming trailing empties.
• Sparse data: builds a sheet with values at C1, E1, A3, B3, E3 and reconstructs the cell map from (rowIdx, colIndex) pairs — verifies column indices reflect actual
  positions, not emission order.
• Nil callback: confirms passing nil is a safe no-op and returns no error.
• Options propagation: sets a styled numeric cell and verifies Options{RawCellValue: true} yields the unformatted raw value through IterateColumns exactly as it does
  through Columns.

Ran the existing TestRows, TestGetRows, and TestRowsIterator to confirm no regression on the shared iterate path.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.60%. Comparing base (6bfbbee) to head (8b76038).
⚠️ Report is 12 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2330   +/-   ##
=======================================
  Coverage   99.60%   99.60%           
=======================================
  Files          32       32           
  Lines       26794    26807   +13     
=======================================
+ Hits        26688    26701   +13     
  Misses         55       55           
  Partials       51       51           

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

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