Return an error when the SQL query contains nested `Explain`

[HaoYang670]

In datafusion, we met a bug (apache/datafusion#4378) which the datafusion-CLI will panic when the query contains nested EXPLAIN.

We fixed this bug using a workaround way, and currently the datafusion will return an Internal error when meeting these invalid explain queries. However the error is generated when converting a logical plan to a physical plan, which is too late.

We'd like the sqlparser to return a parsing error when parsing these invalid query string.
An ideal example is:

> explain explain select 1;
ParseError ...