[5.8] Added SET datatype on MySQL Grammar

[jpedroh]

This pull request adds the MySQL's SET type into the migrations builder. It allows you to make:

    public function up()
    {
        Schema::create('table_name', function (Blueprint $table) {
            $table->bigIncrements('id');
            $table->set("field_name", [ "Possible", "Values" ]);
            $table->timestamps();
        });
    }

That will create a table with a field_name column with the SET dataType. Available only on MySQL.

[huangdijia]

before this, I use https://github.com/huangdijia/laravel-blueprint-extend-set for support set type