Message Title

In the commit #f6aa8ab was added the class MariaDB10Dialect which add few new functions:

public class MariaDB10Dialect extends MariaDB53Dialect {
	public MariaDB10Dialect() {
		registerFunction( "regexp_replace", new StandardSQLFunction( "regexp_replace", StandardBasicTypes.STRING ) );
		registerFunction( "regexp_instr", new StandardSQLFunction( "regexp_instr", StandardBasicTypes.INTEGER ) );
		registerFunction( "regexp_substr", new StandardSQLFunction( "regexp_substr", StandardBasicTypes.STRING ) );
		registerFunction( "weight_string", new StandardSQLFunction( "weight_string", StandardBasicTypes.STRING ) );
		registerFunction( "to_base64", new StandardSQLFunction( "to_base64", StandardBasicTypes.STRING ) );
		registerFunction( "from_base64", new StandardSQLFunction( "from_base64", StandardBasicTypes.STRING ) );
	}

What is funny is that those functions was actually backported to MariaDB from MySQL but the MySQL Dialect doesn't have them.
So I created a small PR to backport those functions from MariaDB dialect to MySQL dialect.

weight_string, to_base64, from_base64: was added in MySQL 5.6 but we have only 5.7 dialect so I added the function there.
regexp_replace, regexp_instr, regexp_substr: was added in MySQL 8