feat: enable timestamps by default in New()

[jamesainslie]

Motivation

Users expect log.New() to behave like log.Default(), but timestamps are omitted. This inconsistency causes confusion since most logging use cases require timestamps for debugging and observability.

See discussion: #191

Summary

Enable timestamps by default in New() and add slog-compatible handler constructors.

Changes

• New() now sets ReportTimestamp: true by default
• Add HandlerOptions struct for slog-style configuration
• Add NewTextHandler() and NewJSONHandler() constructors
• Update existing tests to use NewWithOptions() explicitly

Breaking Change

New() now includes timestamps. Previous behavior available via:

logger := log.NewWithOptions(w, log.Options{})

New API

// slog-compatible constructors
handler := log.NewTextHandler(os.Stderr, &log.HandlerOptions{
    Level:     log.DebugLevel,
    AddSource: true,
})
logger := slog.New(handler)

Closes #191