Tidb grammar online

## Enhancement
I've just added this project grammar to https://mingodad.github.io/parsertl-playground/playground/ (select `Tidb SQL parser (be patient)(partially working)` under examples then click `Parse` to see the parser tree for the content of `Input source` editor).

The main repository is here https://github.com/mingodad/parsertl-playground it's a `Yacc/Lex` runtime grammar evaluator that can provide quick feedback loop when developing/testing/debug grammars.

It can also generate `EBNF` compatible with https://www.bottlecaps.de/rr/ui to generate railroad diagrams see also https://github.com/pingcap/tidb/issues/41195 .

I hope it can help develop/debug/extend/test/document the grammar used in this project.

Any feedback is welcome !