Correct help text and comments for "ScrapTablet" to make sure that it…

* Fix parsing of CAST() statements

CAST() was treated as an alias for CONVERT() but with slightly different
syntax.

This is also described in the documentation at
https://dev.mysql.com/doc/refman/8.0/en/cast-functions.html,
specifically:

With CAST(expr AS type syntax, the CAST() function takes an expression of
any type and produces a result value of the specified type. This operation
may also be expressed as CONVERT(expr, type), which is equivalent. If expr
is NULL, CAST() returns NULL.

This is wrong sadly. CAST() is not equivalent to CONVERT(), specifically
in the context of a CREATE TABLE. For JSON keys, the ARRAY attribute is
possible on a CAST(), but that is not accepted for a CONVERT().

The difference in parsing also shows in MySQL:

```
mysql> select convert(json_keys(c), char(64) array) from t;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'array) from t' at line 1
mysql> select cast(json_keys(c) as char(64) array) from t;
ERROR 1235 (42000): This version of MySQL doesn't yet support 'Use of CAST( .. AS .. ARRAY) outside of functional index in CREATE(non-SELECT)/ALTER TABLE or in general expressions'
```

Here the first statement can't be parsed at all. The second is properly
parsed, but ARRAY is not allowed in the context of CAST() in this
situation (and only in a CREATE TABLE).

This means we should really treat these as two separate expressions and
don't store them both in the same structure. The change here creates a
separate CAST structure, removes the ARRAY option from CONVERT and
updates the grammar and all tests accordingly.

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>

* Handle new cast expression in evalengine and planbuilder

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>

* evalengine: do not duplicate CAST/CONVERT translation

Signed-off-by: Vicent Marti <vmg@strn.cat>

Co-authored-by: Vicent Marti <vmg@strn.cat>
Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>

Co-authored-by: Vicent Marti <vmg@strn.cat>