Add output option for streaming encode results

[flori]

• Added output: parameter to Toon.encode method to allow writing directly to IO objects
• Modified LineWriter to accept optional output argument and write directly to it instead of collecting lines
• Updated resolve_options to include output in the options hash
• Toon.encode now returns nil when output: is specified, otherwise returns the encoded string
• Added tests for the new output: functionality in spec/toon_spec.rb
• LineWriter now uses StringIO as default when no output is provided, maintaining backward compatibility
• Enables memory-efficient streaming of large outputs without keeping entire result in memory