API: nvim_source: fix multiline input

- DOCMD_REPEAT is needed to source all lines of input.
- Fix ":verbose set {option}?" by handling SID_STR in get_scriptname().

closes #8722

src/nvim/api/vim.c

@@ -77,7 +77,7 @@ void nvim_source(String command, Error *err)
   FUNC_API_SINCE(7)
 {
   try_start();
-  do_source_str((char_u *)command.data);
+  do_source_str(command.data);
   try_end(err);
 }