Add comprehensive support for location offsets in issues

[Copilot]

Some linters report character offsets instead of line/column coordinates for issues. This PR adds comprehensive support for offset-based location coordinates to the Cake.Issues library while maintaining full backward compatibility.

Key Features Added

Core Interface Extensions:

• Extended IIssue interface with Offset and EndOffset properties
• Updated IIssueProperty enum with new offset property flags
• Added offset parameters to Issue class constructor with proper validation

Builder Pattern Support:

• Added InFileAtOffset(filePath, offset) method to IssueBuilder
• Added InFileAtOffset(filePath, startOffset, endOffset) method for offset ranges
• Maintains existing fluent API design patterns

Configuration & Conversion:

• Created LocationCoordinates enum with options: LineColumn, Offset, Both
• Added PreferredLocationCoordinates property to IssueProviderSettings
• Implemented LocationCoordinateConverter utility class with methods to convert between line/column ↔ character offsets
• Proper handling of both Unix (\n) and Windows (\r\n) line endings

Usage Examples

// Using offset coordinates with IssueBuilder
var issue = IssueBuilder
    .NewIssue("Syntax error", providerType, providerName)
    .InFileAtOffset("src/Program.cs", 150)  // Character offset 150
    .Create();

// Using offset ranges
var issue2 = IssueBuilder
    .NewIssue("Unused variable", providerType, providerName)  
    .InFileAtOffset("src/Program.cs", 150, 175)  // From offset 150 to 175
    .Create();

// Converting between coordinate systems
var offset = LocationCoordinateConverter.LineColumnToOffset(fileContent, 5, 10);
var (line, col) = LocationCoordinateConverter.OffsetToLineColumn(fileContent, 150);

// Configure preferred coordinate format for an issue provider
var settings = new IssueProviderSettings(logFile);
settings.PreferredLocationCoordinates = LocationCoordinates.Offset;

Backward Compatibility

All existing functionality continues to work unchanged. The new offset properties are optional and existing code that uses line/column coordinates will work exactly as before. Issue providers can now optionally provide offset information alongside or instead of line/column coordinates.

Testing

Added comprehensive test coverage including:

• 26 tests for LocationCoordinateConverter functionality
• 14 tests for IssueBuilder offset methods
• Edge case handling for different line ending formats
• Validation of offset parameter constraints

Fixes #254.

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