Fix(postgres): generate float if the type has precision

[georgesittas]

Postgres doesn't support specifying a precision for the REAL and DOUBLE PRECISION types:

georgesittas=# select 1::real(4);
ERROR:  syntax error at or near "("
LINE 1: select 1::real(4);
                      ^
georgesittas=# select 1::double precision(4);
ERROR:  syntax error at or near "("
LINE 1: select 1::double precision(4);
                                  ^
georgesittas=# select 1::real;
 float4
--------
      1
(1 row)

georgesittas=# select 1::double precision;
 float8
--------
      1
(1 row)

OTOH, according to its docs, it does support a FLOAT(p) type, so we use that instead to (1) roundtrip and (2) facilitate transpilation from other dialects to Postgres:

PostgreSQL also supports the SQL-standard notations float and float(p) for specifying inexact numeric types. Here, p specifies the minimum acceptable precision in binary digits. PostgreSQL accepts float(1) to float(24) as selecting the real type, while float(25) to float(53) select double precision. Values of p outside the allowed range draw an error. float with no precision specified is taken to mean double precision.

Fixes #4508