Smart comment `@nonNull` does not work for computed columns
### Summary

I have a computed column that I would like to mark as non-null using the smart comment feature.

Based on the discussion in https://github.com/graphile/postgraphile/issues/906, the solution is to use the plugin [`@graphile-contrib/pg-non-null`](https://www.npmjs.com/package/@graphile-contrib/pg-non-null).

However, that plugin seems to be mostly abandoned. Quoting from https://github.com/graphile-contrib/pg-non-null/issues/2#issuecomment-851025078:

> Hey there, this plugin is a bit obsolete. [The `@notNull` smart comment is available in PostGraphile as a built-in.](https://www.graphile.org/postgraphile/smart-tags/#notnull) This plugin is mostly useful for its `PgNonNullRelationsPlugin` part.

Unfortunately, the built-in support for `@nonNull` does not seem to work for computed columns :(

<!-- A brief summary of the problem. -->

### Steps to reproduce

Using a slightly modified example from https://github.com/graphile-contrib/pg-non-null#usage

```sql
CREATE TABLE public.customers (
  id         serial PRIMARY KEY,
  first_name text NOT NULL,
  last_name  text
);

CREATE FUNCTION public.customers_full_name("user" public.customers) RETURNS text AS
$$
  SELECT "user".first_name || ' ' || "user".last_name
$$
LANGUAGE SQL STABLE;
COMMENT ON FUNCTION public.customers_full_name(public.customers) IS E'@nonNull';
```

```shell
$ postgraphile -c postgres://YOUR_CONNECTION_STRING --export-schema-graphql schema.graphql
```

### Expected results

The generated GraphQL schema marks the field `Customers.fullName` as non-nullable.

```graphql
type Customer implements Node {
  """
  A globally unique identifier. Can be used in various places throughout the system to identify this single value.
  """
  nodeId: ID!
  id: Int!
  firstName: String!
  lastName: String
  fullName: String!
}
```

### Actual results

The generated GraphQL schema marks the field `Customers.fullName` as nullable.

```graphql
type Customer implements Node {
  """
  A globally unique identifier. Can be used in various places throughout the system to identify this single value.
  """
  nodeId: ID!
  id: Int!
  firstName: String!
  lastName: String
  fullName: String
}
```

### Additional context

To get the desired behaviour, I have to configure the `@graphile-contrib/pg-non-null` plugin.

```shell
$ postgraphile -c postgres://YOUR_CONNECTION_STRING --export-schema-graphql schema.graphql --append-plugins "@graphile-contrib/pg-non-null"
```

### Possible Solution

It would be awesome if Postgraphile supported this feature out of the box so that I don't have to install that plugin.

The docs at https://www.graphile.org/postgraphile/why-nullable/#what-about-computed-fields say:

> I'd be happy to accept a Pull Request that adds functionality marking a function as non-nullable via a smart comment (e.g. `COMMENT ON FUNCTION foo_func(foo) IS E'@nonNull';`) - do raise an issue if this is of interest to you.

I am happy to contribute to this feature if you could provide me with a few pointers on where to start looking in the codebase and where/how to add tests.