[FEATURE] SCORE() function to return score instead of _score

[acarbonetto]

Is your feature request related to a problem?
Following the below issues, after adding score() function from the legacy engine (doc), adding PPL and SQL syntax that allows the user to include a function without arguments in place of the _score identifier.

#639
#432

What solution would you like?
The existing SCORE() function requires a relevance search function as the first argument (with an optional boost for the second argument) without really needing one and the boost feature is really irrelevant.

A better solution would be to include a score() function (without arguments) which is synonymous with _score identifier but also allows the user to include the score() function instead of using _score meta-field identifier.

Example:

SELECT SCORE(), * FROM library WHERE MATCH(name, 'dune') ORDER BY SCORE() DESC;

What alternatives have you considered?
This function would be easier to use than the existing SCORE(search, boost) that is already available.

Do you have any additional context?
Add any other context or screenshots about the feature request here.