Simplify the builder structure

[zoedsoupe]

Problem

postgres-js and other implementations that relies on OOP languages separate operations in a logical way, being delimited by the called "builders", which remains 3:

• QueryBuilder: select, insert, update, ...
• FilterBuilder: match, eq, neq, range, ...
• TransformBuilder: limit, single, ...

That makes sense when you can chain method to a single structure, however in elixir data is immutable and state is explicit, so we do not have methods that can mutate state but instead function that modify the state (given as param) and return a new copy of this state, changed.

I could split the main module Supabase.PostgREST

Solution

Split the Supabase.PostgREST into:

• Supabase.PostgREST.FilterBuilder
• Supabase.PostgREST.QueryBuilder
• Supabase.PostgREST.TransformBuilder

The Supabase.PostgREST function nows delegates all functions to these submodules, so we don't break the main API

Rationale

Closes #5