[10.x] Support providing subquery as value to where builder method

[gdebrauwer]

Currently, it is impossible to provide a subquery as the $value in a where builder method. You can only achieve a where with a subquery by providing the subquery as the first parameter. When doing that, you must also wrap the column name in DB::raw() otherwise the Builder will think the column name is just a raw value. When you use an operator, you also have to reverse that operator, which makes the code less readable/logical in my opinion.

User::where(
    Game::select('min_score')->whereColumn('games.id', 'users.game_id'),
    '<',
    DB::raw('score')
)

This PR fixes that, so you can just provide the column name first, and then provide the subquery as the “value” parameter. That makes the code cleaner and more readable.

User::where(
    'score',
    '>',
    Game::select('min_score')->whereColumn('games.id', 'users.game_id'),
)

I targeted the 10.x branch because I think changing the parameter signature of a protected method is not a breaking change, but I'm not sure. If that is regarded as a breaking change, I will create a new PR against the master branch.