feat: add encodeFile() and encodeDocumentFile() convenience methods

[dereuromark]

Summary

Add file writing convenience methods to the Toml facade:

• encodeFile(path, data, options): Encode PHP array to TOML and write to file
• encodeDocumentFile(path, doc, options): Encode AST document to TOML and write to file

Usage

// Encode array directly to file
Toml::encodeFile('/path/to/config.toml', $data);

// With options
Toml::encodeFile('/path/to/config.toml', $data, new EncoderOptions(sortKeys: true));

// Encode document to file (e.g., after AST editing)
$doc = Toml::parse($input, true);
// ... modify $doc ...
Toml::encodeDocumentFile('/path/to/output.toml', $doc, new EncoderOptions(
    documentFormatting: DocumentFormattingMode::SourceAware
));

Details

• Both methods throw EncodeException on write failure
• Consistent with existing decodeFile() pattern
• All encoder options supported
• Full test coverage including error cases