inFullTextOrder() method to sort full-text queries by rank

[gbradley]

8.79 added fulltext support for MySQL and Postgres via the whereFullText() and orWhereFullText() methods. However when doing a natural language search you often want to sort the returned results by rank, but currently there appears no way to do this, leaving us to build the query manually depending on the underlying DB.

Ideally an inFullTextOrder($alias) method would abstract this away (named to keep convention with the existing inRandomOrder() method). I have essentially implemented this for MySQL:

public function inFullTextOrder($as = 'rank', $direction = 'desc')
{  

    $where = Arr::first($this->wheres, fn($where) => $where['type'] == 'Fulltext');
    $query = $this->grammar->whereFullText($this, $where);

    if ($this->columns === null) {
        $this->select('*');
    }

    $this->selectRaw(
        '('.$query.') as '.$this->grammar->wrap($as), [$where['value']]
    );
    $this->orderBy($as, $direction);
}

however Postgres (which I'm not familiar with) obviously requires a different syntax. Happy to attempt and do a PR if useful / the right way to go.

[lloricode]

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