LibJS: Avoid unnecessary SourceRange construction in parse_program()

This takes `test-js` runtime from 4.3 sec to 4.1 sec on my machine.

Userland/Libraries/LibJS/Parser.cpp

@@ -497,7 +497,7 @@ NonnullRefPtr<Program> Parser::parse_program(bool starts_in_strict_mode)
     else
         parse_module(program);
 
-    program->source_range().end = position();
+    program->set_end_offset({}, position().offset);
     return program;
 }