fix: add RenderMessage() method for custom sinks (#64)

[willibrandon]

Description

Adds a public RenderMessage() method to core.LogEvent to allow custom sinks to properly render message templates. This fixes issue #64 where custom sinks had to use manual string replacement that failed with destructuring operators ({@Property}), scalar hints ({$Property}), and format specifiers ({Property:format}).

The implementation follows Serilog's approach by providing a public API that handles all template rendering features. Also includes a refactor of internal/parser/token.go to resolve an import cycle by using interface-based design.

Type of change

• Bug fix
• New feature
• Performance improvement
• Documentation update

Checklist

• Tests pass (go test ./...)
• Linter passes (golangci-lint run)
• Benchmarks checked (if performance-related)
• Documentation updated (if needed)
• Zero-allocation promise maintained (if applicable)

Additional notes

• Fixes Custom sinks cannot render message templates - internal/parser is not accessible #64
• Added test coverage in core/event_test.go
• Created working example in examples/custom-sink/main.go
• Updated README.md and CHANGELOG.md with the new feature
• Refactored parser to use fmt.Stringer interface instead of concrete types to break import cycle

[Copilot]

Pull Request Overview

Adds a public RenderMessage() method to core.LogEvent to allow custom sinks to properly render message templates, fixing issue #64 where custom sinks failed with advanced template features.

• Added RenderMessage() method to core.LogEvent with proper error handling and fallback
• Refactored token parser to use fmt.Stringer interface instead of concrete types to resolve import cycle
• Created comprehensive example demonstrating custom sink usage with all template features

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
core/event.go	Added public RenderMessage() method with documentation and error handling
internal/parser/token.go	Refactored to use fmt.Stringer interface and removed import dependency
core/event_test.go	Added comprehensive tests for all template rendering features
examples/custom-sink/main.go	Created working example demonstrating custom sink with RenderMessage()
README.md	Updated custom sink documentation with new RenderMessage() usage
CHANGELOG.md	Documented the new feature and its capabilities

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