Separate pg shorthand in sql/pg

[lucleray]

The goal of this PR is to make the postgres shorthand "opt-in".

Before :

const sql = require('@sequencework/sql')
const movies = await sql(db)`select * from movies`

After :

const sql = require('@sequencework/sql/pg') // ⚠️we import @sequencework/sql/pg
const movies = await sql(db)`select * from movies`

IMO, it also makes the code cleaner.

[yannvgn]

Sounds good to me. As a next step, it might be interesting to refine this separation to figure out how to support other clients (and their query format).

But let's keep it simple for the moment 👌