[9.x] Added a way to retrieve the first column of the first row from a query

[peterlupu]

Currently there is no clean way to retrieve the first column of the first row of a more complex raw query.
For example

SELECT COUNT(CASE WHEN food = 'burger' THEN 1 END) FROM menu_items;

The quickest way I know, without using the Query Builder's value() method is:

DB::selectOne("SELECT COUNT(CASE WHEN food = 'burger' THEN 1 END) AS burgers FROM menu_items;")->burgers

This PR adds DB::scalar(), so you can retrieve the result without aliasing the column and referencing the alias:

DB::scalar("SELECT COUNT(CASE WHEN food = 'burger' THEN 1 END) FROM menu_items;")
// returns the count directly

If multiple columns are selected, an exception is thrown (MultipleColumnsSelectedException).