PostgreSQL Compatibility: boolean casting

In `bool.html` we mention that boolean values can be cast to numeric types. I think we should also mention in `postgresql-compatibility.html#features-that-differ-from-postgresql` that this is an extension of what PostgreSQL allows (they only allow casting between BOOL and INTEGER / INT / INT4, not any of the other numeric types).

Here's a demonstration. PostgreSQL 14.2:

```
michae2=# SELECT true::INT2;
2022-03-29 15:50:58.483 PDT [59188] ERROR: cannot cast type boolean to smallint at character 12
2022-03-29 15:50:58.483 PDT [59188] STATEMENT: SELECT true::INT2;
ERROR: cannot cast type boolean to smallint
LINE 1: SELECT true::INT2;
^

michae2=# SELECT true::INT4;
int4
------
1
(1 row)

michae2=# SELECT true::INT8;
2022-03-29 15:51:04.366 PDT [59188] ERROR: cannot cast type boolean to bigint at character 12
2022-03-29 15:51:04.366 PDT [59188] STATEMENT: SELECT true::INT8;
ERROR: cannot cast type boolean to bigint
LINE 1: SELECT true::INT8;
^

michae2=# SELECT true::FLOAT4;
2022-03-29 15:51:08.538 PDT [59188] ERROR: cannot cast type boolean to real at character 12
2022-03-29 15:51:08.538 PDT [59188] STATEMENT: SELECT true::FLOAT4;
ERROR: cannot cast type boolean to real
LINE 1: SELECT true::FLOAT4;
^

michae2=# SELECT true::FLOAT8;
2022-03-29 15:51:11.104 PDT [59188] ERROR: cannot cast type boolean to double precision at character 12
2022-03-29 15:51:11.104 PDT [59188] STATEMENT: SELECT true::FLOAT8;
ERROR: cannot cast type boolean to double precision
LINE 1: SELECT true::FLOAT8;
^

michae2=# SELECT true::DECIMAL;
2022-03-29 15:51:15.567 PDT [59188] ERROR: cannot cast type boolean to numeric at character 12
2022-03-29 15:51:15.567 PDT [59188] STATEMENT: SELECT true::DECIMAL;
ERROR: cannot cast type boolean to numeric
LINE 1: SELECT true::DECIMAL;
^
```

CockroachDB 22.1:

```
demo@127.0.0.1:26257/defaultdb> SELECT true::INT2;
int2
--------
1
(1 row)

Time: 0ms total (execution 0ms / network 0ms)

demo@127.0.0.1:26257/defaultdb> SELECT true::INT4;
int4
--------
1
(1 row)

Time: 1ms total (execution 0ms / network 0ms)

demo@127.0.0.1:26257/defaultdb> SELECT true::INT8;
int8
--------
1
(1 row)

Time: 0ms total (execution 0ms / network 0ms)

demo@127.0.0.1:26257/defaultdb> SELECT true::FLOAT4;
float4
----------
1
(1 row)

Time: 1ms total (execution 0ms / network 0ms)

demo@127.0.0.1:26257/defaultdb> SELECT true::FLOAT8;
float8
----------
1
(1 row)

Time: 1ms total (execution 0ms / network 0ms)

demo@127.0.0.1:26257/defaultdb> SELECT true::DECIMAL;
numeric
-----------
1
(1 row)

Time: 1ms total (execution 0ms / network 0ms)
```