* feat: improve keywords definations and add prefix named KW_
* test: capitalize keywords in alter sql test cases
* test: capitalize keywords in create sql test cases
* feat: merge interval rule and timeIntervalExpression rule in flinksql grammar
* test: put keywords in quotation marks around insert sql test cases
* test: put keywords in quotation marks around select sql test cases
* feat: improve uid rule
* test: rename sum to _sum because it is a reserved keyword
* docs: improve comments in flinkSqlLexer