bug(oracle): incorrect data type mapping for Type.TIMESTAMPTZ

[gforsyth]

Before you file an issue

[ins] In [4]: sg.__version__
Out[4]: '18.7.1.dev519'

Fully reproducible code snippet
Please include a fully reproducible code snippet or the input sql, dialect, and expected output.

The following should round-trip in Oracle. TIMESTAMPTZ is not a valid type in Oracle.

[ins] In [1]: import sqlglot as sg

[ins] In [2]: expr = sg.parse_one("TIMESTAMP (3) WITH TIME ZONE", read="oracle")

[ins] In [3]: expr.sql(dialect="oracle")
Out[3]: 'TIMESTAMPTZ(3)

Official Documentation
Please include links to official SQL documentation related to your issue.
https://docs.oracle.com/en/database/oracle/oracle-database/23/sqlrf/Data-Types.html#GUID-7B72E154-677A-4342-A1EA-C74C1EA928E6