ast.rb: RubyVM::AST.parse and .of accepts save_script_lines: true

This option makes the parser keep the original source as an array of
the original code lines. This feature exploits the mechanism of
`SCRIPT_LINES__` but records only the specified code that is passed to
RubyVM::AST.of or .parse, instead of recording all parsed program texts.

node.h

@@ -399,6 +399,7 @@ typedef struct rb_ast_body_struct {
     const NODE *root;
     VALUE compile_option;
     int line_count;
+    VALUE script_lines;
 } rb_ast_body_t;
 typedef struct rb_ast_struct {
     VALUE flags;