[FEATURE]: column.as("alias") method

### Describe what you want

currently, the `Column` type has no `as` method, so I use `column.getSQL().mapWith(column.mapFromDriverValue).as('alias')` to properly select a column with alias.

```ts
const result = await db.select({
sellerId: user.id.getSQL().mapWith(user.id.mapFromDriverValue).as('sellerId')
})
```

It would be nice to be able to just do it with:

```ts
const result = await db.select({
sellerId: user.id.as('sellerId')
})
```