sql2dbml - Mysql 8.0.13+ reference to column in DEFAULT generates incorrect dbml

I ran into the issue that for sql statements like the following
```sql
CREATE TABLE `some_table` (
code VARCHAR(6)
);

-- migration file later on
UPDATE TABLE `some_table`
ADD name VARCHAR(80) DEFAULT(`code`);
```
generates incorrect dbml when generated from an sql dump. The resulting dbml will have double backticks around the word `code`, as follows:

```dbml
name VARCHAR(80) DEFAULT(``code``)
```

This leads to tools following in the pipeline to error.