Returns an aliased version of the function.

In addition to slapping as "the_alias" to the end of the SQL, this method also provides strict typing:

const result = await db
  .selectFrom('person')
  .select(
    (eb) => eb.fn.count<number>('id').as('person_count')
  )
  .executeTakeFirstOrThrow()

// `person_count: number` field exists in the result type.
console.log(result.person_count)

The generated SQL (PostgreSQL):

select count("id") as "person_count"
from "person"