rubocop-ast: Add type for RuboCop::AST::ProcessedSource#lines (#1019)

ref: https://github.com/rubocop/rubocop-ast/blob/v1.46.0/lib/rubocop/ast/processed_source.rb#L77

Author: tk0miya

gems/rubocop-ast/1.46/rubocop-ast.rbs

@@ -7,6 +7,7 @@ module RuboCop
       attr_reader parser_engine: Symbol
 
       def initialize: (String source, Float ruby_version, ?path?, ?parser_engine: Symbol, ?prism_result: untyped) -> untyped
+      def lines: () -> Array[String]
       def raw_source: () -> String
       def buffer: () -> Parser::Source::Buffer
       def comments: () -> Array[Parser::Source::Comment]