Add default logger instance and package-level API

[paganotoni]

This PR introduces a default logger instance and package-level logging functions, making it easier to use the logger package without explicitly creating a logger instance.

New API

Default Logger

• logger.Default() - Returns the default FieldLogger instance (initialized with InfoLevel on first call)
• logger.SetDefault(l FieldLogger) - Sets a custom default logger

Package-Level Functions

All logging methods are now available as package-level functions that use the default logger:

Non-formatted:

• logger.Debug(args ...any)
• logger.Info(args ...any)
• logger.Warn(args ...any)
• logger.Error(args ...any)
• logger.Fatal(args ...any)
• logger.Panic(args ...any)

Formatted:

• logger.Debugf(msg string, args ...any)
• logger.Infof(msg string, args ...any)
• logger.Printf(msg string, args ...any)
• logger.Warnf(msg string, args ...any)
• logger.Errorf(msg string, args ...any)
• logger.Fatalf(msg string, args ...any)

With fields:

• logger.WithField(key string, value any) FieldLogger
• logger.WithFields(fields map[string]any) FieldLogger

Usage Examples

// Simple usage - no need to create a logger
logger.Info("Server starting")
logger.WithField("port", 8080).Info("Server started")
logger.Errorf("Failed to connect: %v", err)

// Or get the default logger and use it directly
log := logger.Default()
log.Info("Using default logger")

// Set a custom default logger
customLogger := logger.New(logger.DebugLevel)
logger.SetDefault(customLogger)

Code Organization

• Moved fieldLogger struct and methods to field.go for better separation of concerns
• Default logger functions remain in default.go

Tests

Added comprehensive tests covering:

• Default logger functionality (Default(), SetDefault())
• All package-level wrapper functions
• Field logging with WithField/WithFields
• Level filtering behavior
• The logrus subpackage (27 tests)
• The textFormatter implementation (9 tests)

Total: 74 tests across both packages