Add general parser checks for top-level only tokens (i.e. DML and DDL ops)

[alancai98]

While working on adding stored procedures calls (#345), which we limited to just top-level queries, I couldn't find any existing top-level token checks. We should add parser checks to prohibit using top-level tokens outside of the top-level. Currently, the parser won't throw an exception when mixing SFW queries with DML and DDL operations. So currently, a query such as SELECT * FROM DELETE FROM foo would not have a parser exception.

[abhikuhikar]

#373

[abhikuhikar]

This has been addressed by the PR.