Add custom value formatters for dates, numbers, and currency

[vaceslav]

Description

Extend the format specifier system to support custom formatting for dates, numbers, and currency values.

Current State

• Boolean formatters implemented (:checkbox, :yesno, etc.)
• Basic architecture for format specifiers exists
• Need to extend to other data types

Proposed Formatters

Date Formatting

{{BirthDate:yyyy-MM-dd}}      → 2025-01-15
{{DueDate:MMM dd, yyyy}}       → Jan 15, 2025
{{CreatedAt:relative}}         → 2 days ago
{{UpdatedAt:iso}}              → 2025-01-15T10:30:00Z

Number Formatting

{{Price:N2}}                   → 1,234.56
{{Percentage:P1}}              → 45.3%
{{Count:N0}}                   → 1,234
{{Scientific:E2}}              → 1.23E+03

Currency Formatting

{{Amount:C}}                   → ,234.56 (culture-aware)
{{Amount:C:EUR}}               → €1,234.56
{{Amount:C:GBP}}               → £1,234.56

Custom Formatters

var options = new DocumentProcessorOptions
{
    CustomFormatters = new Dictionary<string, IValueFormatter>
    {
        ["phone"] = new PhoneFormatter(),
        ["ssn"] = new SocialSecurityFormatter(),
        ["uppercase"] = new UpperCaseFormatter()
    }
};

Technical Design

IValueFormatter Interface

public interface IValueFormatter
{
    bool CanFormat(Type valueType, string format);
    string Format(object value, string format, CultureInfo culture);
}

Built-in Formatters

• DateTimeFormatter
• NumberFormatter
• CurrencyFormatter
• StandardFormatter (.NET format strings)

Culture Support

var options = new DocumentProcessorOptions
{
    Culture = new CultureInfo("de-DE"),
    // {{Amount:C}} → 1.234,56 € (German format)
};

Use Cases

• Financial documents with currency
• Reports with dates in specific formats
• Multi-language document generation
• Scientific/technical documents
• Invoices and receipts

Acceptance Criteria

• Date formatting with standard .NET format strings
• Number formatting with standard .NET format strings
• Currency formatting with culture support
• Custom formatter registration
• Culture-aware formatting
• Fallback for invalid formats
• Comprehensive tests
• Documentation and examples

Priority

Medium - Highly requested feature

Backwards Compatibility

• Fully backwards compatible
• Format specifiers are optional
• Existing templates continue to work

References

• .NET format strings
• Issue Feature: Add format specifiers for boolean values (checkbox, yes/no, etc.) #1 (Boolean formatters - already implemented)