sql: add support for negative scale in decimals

Postgres allows specifying a negative scale for decimal types. While a positive/zero scale specifies a point _after_ the decimal point to round values, a negative scale specifies a point _before_ the decimal point. Currently, CRDB does not allow specifying a negative scale:
```
root@localhost:26257/defaultdb> select 1::decimal(2, -3);
ERROR: statement ignored: at or near "-": syntax error
SQLSTATE: 42601
DETAIL: source SQL:
select 1::decimal(2, -3)
^
root@localhost:26257/defaultdb> create table t (x decimal(2, -3));
ERROR: statement ignored: at or near "-": syntax error
SQLSTATE: 42601
DETAIL: source SQL:
create table t (x decimal(2, -3))
^
HINT: try \h CREATE TABLE
```
Relevant Postgres docs: https://www.postgresql.org/docs/current/datatype-numeric.html#DATATYPE-NUMERIC-DECIMAL

Jira issue: CRDB-57538