Add R1C1 formula notation support

[loidolt]

Description

Currently, GitCells only extracts formulas in A1 notation. R1C1 notation is not supported because the underlying excelize library doesn't expose this functionality.

Current Behavior

// internal/converter/types.go:97-99
// TODO: Extract R1C1 formula if supported by excelize
// Currently excelize doesn't have direct R1C1 support
formulaR1C1 := ""

Desired Behavior

Extract and preserve both A1 and R1C1 formula notations for complete Excel fidelity.

Potential Solutions

1. Wait for excelize to add R1C1 support
2. Implement custom R1C1 conversion from A1 notation
3. Parse Excel XML directly to extract R1C1 formulas

Impact

• Low priority: Most users work with A1 notation
• Would improve compatibility with Excel macros and advanced features
• Nice-to-have for complete Excel round-trip fidelity

Labels

enhancement, converter