[6.x] Fix plucking column name containing a space

[shadoWalker89]

This PR fixes this #31298

Plucking a column that contains a space in it's name will fail

NoteRow::pluck('Identifiant groupe');

Will give this error

ErrorException : Undefined property: stdClass::$groupe`
 W:\projects\exam-laravel\src\exam-laravel\vendor\laravel\framework\src\Illuminate\Database\Query\Builder.php:2330
 W:\projects\exam-laravel\src\exam-laravel\vendor\laravel\framework\src\Illuminate\Database\Query\Builder.php:2302
 W:\projects\exam-laravel\src\exam-laravel\vendor\laravel\framework\src\Illuminate\Database\Eloquent\Builder.php:687

As you can see in the error the First word of the column name is striped Identifiant and only the second word groupe is being used.
The problem is that the Illuminate\Database\Query\Builder::stripTableForPluck() is splitting the column name by space to allow for plucking while using column alias.
This behavior was introduced in this commit 4fd6db1

The PR preserves the alias behavior and fixes column names with a space.
This PR changes the alias behavior from splitting by space to splitting by as keyword. It also takes into consideration that the developper could write AS in uppercase.
Existing tests are passing a new test was added to confirm that plucking columns with a space in the name works as well.

As for why i have space in the column names; It's due to me developing a new application based on a legacy app database and both should exist at the same time until porting the features of the legacy one is over.