[8.x] Add dropColumns method on the schema class

[imanghafoori1]

This thin wrapper, adds a little bit of syntactic sugar for down methods in migrations:

refactor this:

public function down()
{
    Schema::table('posts', function (Blueprint $table) {
        $table->dropColumn('user_id');
        $table->dropColumn('state');
    });
}

into:

public function down()
{
    Schema::dropColumns('posts', ['user_id', 'state']);
}

• Since the Illuminate\Database\Schema\Builder class is Not macroable, it is not very like to break something.
  The only possible case is, if someone has extended the Builder class, and has the same method name in the subclass but with a different signature, which is very unlikely.

• If accepted, I will add tests in another PR.

[imanghafoori1]

the method signature can also be like this:

public function dropColumns($table, ...$columns) 
{
    ...
}

to allow :

public function down()
{
    Schema::dropColumns('projects', 'user_id', 'state');
}

but I do not think it is very a pleasant API.