feat(octetcounting,nontransparent): restore backward compatibility while keeping WithMachineOptions

[leodido]

Summary

This PR restores backward compatibility with v4.2.0 by re-adding support for the syslog.WithBestEffort() API, while preserving all improvements introduced in PR #23 (syslog.WithMachineOptions()).

Problem

PR #23 introduced breaking changes by:

• Removing BestEfforter from the Parser interface
• Removing the syslog.WithBestEffort() generic option
• Requiring users to migrate to syslog.WithMachineOptions(rfc5424.WithBestEffort())

While this is a useful pattern in some scenarios, it would have forced a major version bump to v5.0.0 and broken existing user code.

Solution

Support both APIs simultaneously by:

1. Re-adding BestEfforter to the Parser interface
2. Restoring the syslog.WithBestEffort() generic option
3. Converting the legacy bestEffort flag to a MachineOption internally
4. Allowing both approaches to coexist without conflicts

How It Works

The old bestEffort bool flag is maintained for backward compatibility. When set via the old API, it's automatically converted to the appropriate MachineOption during parser construction:

// If bestEffort flag was set via old API, add it to internalOpts
if p.bestEffort {
    p.internalOpts = append(p.internalOpts, rfc5424.WithBestEffort())
}

This means both APIs produce identical behavior:

Old API (backward compatible):

parser := nontransparent.NewParser(syslog.WithBestEffort())

New API (from PR #23):

parser := nontransparent.NewParser(
    syslog.WithMachineOptions(rfc5424.WithBestEffort())
)

Mixed (also works):

parser := nontransparent.NewParser(
    syslog.WithBestEffort(),
    syslog.WithMachineOptions(rfc3164.WithYear(rfc3164.CurrentYear{}))
)

Benefits

• ✅ No breaking changes - existing code continues to work
• ✅ Preserves PR feat!: allow passing down machine options from nontransparent/octetcounting parsers #23 functionality - new WithMachineOptions API fully functional
• ✅ Smooth migration path - users can migrate at their own pace
• ✅ Version 4.3.0 instead of 5.0.0
• ✅ Comprehensive tests - both APIs verified to produce identical results

Changes

• syslog.go - Re-added BestEfforter to Parser interface
• options.go - Restored WithBestEffort() generic option
• nontransparent/parser.go.rl - Added backward compatibility logic
• nontransparent/parser.go - Regenerated from ragel source
• octetcounting/parser.go - Added backward compatibility logic
• *_test.go - Added comprehensive tests for both APIs

Testing

All existing tests pass, plus new tests verify:

• Old API works correctly
• New API works correctly
• Both APIs produce identical results
• Mixed usage works correctly