Support single-argument version for merge table function

Why it matters

This feature simplifies the syntax for the merge table function by enabling users to specify a single argument consisting of an optional database name and a tables regular expression. It reduces complexity when merging tables matching a pattern from a specific database, making it more convenient and concise.

How to use it

Users can call the merge table function with a single argument combining an optional database name and a tables regular expression as follows:

SELECT * FROM merge(['db_name', ] 'tables_regexp')

This new syntax supports either specifying just the tables_regexp or including the optional db_name as the first element in the array.