[9.x] Introduce annotated builder contracts

[inxilpro]

This is a revised version of #37956 that also brings in the new tests from #40402. Instead of adding any methods to the interface, I've added @mixin annotations. I've also added two contracts to address the differences between Eloquent queries and base queries.

As a result, you can do the following:

use Illuminate\Contracts\Database\Query\Builder;

return Model::query()
  ->whereNotExists(function(Builder $query) {
    // no need to know that $query is a Query\Builder
  })
  ->whereHas('relation', function(Builder $query) {
    // no need to know that $query is an Eloquent\Builder
  })
  ->with('relation', function(Builder $query) {
    // no need to know that $query is an Eloquent\Relation
  });

If you need to use Eloquent-specific features, then you need to type-hint the Eloquent builder:

use Illuminate\Contracts\Database\Eloquent\Builder as Eloquent;
use Illuminate\Contracts\Database\Query\Builder;

return Model::query()
  ->whereNotExists(function(Builder $query) {
    // only do base builder operations here
  })
  ->whereHas('relation', function(Eloquent $query) {
    // maybe I need another whereHas() here
  });

The @mixin annotations provide good IDE support without needing to change the existing Builders at all:

I hope this works out as a good compromise for the 9.x release!