Parser: Optionally warn about missing AS for column and table aliases

Add a new GUC "missing_as_warning" (defaults to false, the previous
behavior) to raise a WARNING when a column or table alias is used without
the AS keyword.

This allows catching some types of errors where another keyword or a comma
was missing and a label is being used as an alias instead of something else,
for example cases like:

    SELECT COUNT(*) files;
    SELECT * FROM files users;
    SELECT path size FROM files INTO f_path, f_size;