Support json_agg(column_ref)

[SimonSimCity]

Fixes #1280.

Wow, my first PR in this repo 🤩

If there was something wrong about what I wrote - e.g. if the types are too narrow or wide, please let me know.

Here's what I know about this:

• Postgresql returns an array if there are elements in the linked table.
• Postgresql returns null otherwise.

According to the docs, this method does only exist for Postgresql, and I therefore didn't look into any other of the options.

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Open in Stackblitz • kysely_koa_example

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commit: 6be68bc

[SimonSimCity]

Since it's likely that people will use this functionality with coalesce, I thought of adding a helper function, similar to the existing jsonArrayFrom. I don't know how you have it in regards to other dialects, because I would only be able to add it for Postgres. Here's an example of how this could be used: #1280 (comment)