Allow whitespace around database comments for smart tags
### Feature description

<!-- Please describe the feature you'd like to see in as much detail as you
      can. -->
Writing smart tags via database comments is often tricky and involves a lot of trial-and-error. For me, most of that involves trying out different styles of formatting. I'd like to be able to format the comment however I want and still achieve the desired outcome.

### Motivating example

<!-- What motivated this feature request? Why is this feature useful to other
      people? What other use cases can you think of for it?  -->
A bit of time lost trying to get multiple smart tags on the same column to work. What ended up working was:

```sql
COMMENT
  ON COLUMN "foo"."bar"
  IS E'
@name "baz"
@notNull
@omit create,update';

-- OR:

COMMENT
  ON COLUMN "foo"."bar"
  IS E'@name "baz"\n@notNull\n@omit create,update';
``` 

How I'd have liked to format the comment:

```sql
COMMENT
  ON COLUMN "foo"."bar"
  IS E'
    @name "baz"
    @notNull
    @omit create, update'; -- including whitespace between commas here
``` 

### Breaking changes

<!-- If this feature requires breaking changes, detail them here, otherwise
      delete this section -->
N/A

### Supporting development

I [tick all that apply]:

<!-- To tick a checkbox, change it from `[ ]` to `[x]` -->

- [x] am interested in building this feature myself - could someone please point me at where to start? :)
- [x] am interested in collaborating on building this feature
- [x] am willing to help testing this feature before it's released
- [x] am willing to write a test-driven test suite for this feature (before it exists)
- [ ] am a [Graphile sponsor](https://www.graphile.org/sponsor/) ❤️
- [ ] have an active [support or consultancy contract](https://www.graphile.org/support/) with Graphile
