Add pg_basetype() function to obtain a DOMAIN base type

Hello hackers,

Currently obtaining the base type of a domain involves a somewhat long
recursive query. Consider:

```
create domain mytext as text;
create domain mytext_child_1 as mytext;
create domain mytext_child_2 as mytext_child_1;
```

To get `mytext_child_2` base type we can do:

```
WITH RECURSIVE
recurse AS (
SELECT
oid,
typbasetype,
COALESCE(NULLIF(typbasetype, 0), oid) AS base
FROM pg_type
UNION
SELECT
t.oid,
b.typbasetype,
COALESCE(NULLIF(b.typbasetype, 0), b.oid) AS base
FROM recurse t
JOIN pg_type b ON t.typbasetype = b.oid
)
SELECT
oid::regtype,
base::regtype
FROM recurse
WHERE typbasetype = 0 and oid = 'mytext_child_2'::regtype;

oid | base
----------------+------
mytext_child_2 | text
```

Core has the `getBaseType` function, which already gets a domain base type
recursively.

I've attached a patch that exposes a `pg_basetype` SQL function that uses
`getBaseType`, so the long query above just becomes:

```
select pg_basetype('mytext_child_2'::regtype);
pg_basetype
-------------
text
(1 row)
```

Tests and docs are added.

Best regards,
Steve Chavez