[7.x] Add the ability to create indexes as expressions

[reinink]

This PR adds the ability to create indexes as expressions. Previously (to the best of my knowledge), the only way to do this was using DB::statement(). For example:

public function up()
{
    Schema::create('users', function (Blueprint $table) {
        $table->id();
        $table->string('name');
        $table->date('birth_date')->nullable();
        $table->timestamps();
    });

    DB::statement('ALTER TABLE users ADD INDEX birthday_index ((date_format(birth_date, "%m-%d")))');
}

Not only does this look awful, it also doesn't take advantage of any platform specific grammar (should the expressions themselves be cross platform, of course).

This PR adds a new rawIndex() method to the Blueprint class, which allows for a much more Laravel-style syntax for creating expression indexes. For example:

public function up()
{
    Schema::create('users', function (Blueprint $table) {
        $table->id();
        $table->string('name');
        $table->date('birth_date')->nullable();
        $table->timestamps();

        $table->rawIndex('(date_format(birth_date, "%m-%d"))', 'birthday_index');
    });
}

The other method name I considered was $table->expressionIndex(), but I felt this was probably more appropriate, since it's essentially using a raw expression under the hood.

This method requires an index name as the second argument, since there isn't a nice way to autogenerate the name.

I've tested this on MySQL, Postgres, and SQLite, and it works great. I haven't tested on it on SQL Server (no access to it), but I'm pretty confident it will work.

Compatibility:

• Postgres has had functional index support since version 7.2 - (2002).
• SQLite has had functional index support since version 3.9.0 - (2015).
• MySQL has had functional index support since version 8.0.13 - (2018).