[PLSql] DATE_FORMAT - valid constant name

[frolovgo]

I have faced with parsing errors during testing my plsql linter on real codebase and prepared minor fix for plsql grammar

[frolovgo]

Most issues are already fixed in #4808
Only one remains:
example(PL/SQL procedure successfully completed):

declare
    DATE_FORMAT constant varchar2(100) := 'yyyy-mm-dd hh24:mi:ss';
begin
    null;
end;