Implement optional lengths for string types

MariaDB currently requires string lengths to be specified, for example:

CREATE TABLE t1 (

    a VARCHAR(10),

    b VARCHAR(20)

);

The 2023 SQL standard no longer requires this, so:

CREATE TABLE t1 (

    a VARCHAR,

    b VARCHAR

);

would be valid code. The limit is then left to the implementation to define, for example in PostgreSQL, characters without lengths become character(1), while character varying without lengths accept strings of any size.