Add support for DENY statements

[aharpervc]

This is another statement for SQL Server: https://learn.microsoft.com/en-us/sql/t-sql/statements/deny-transact-sql, but implemented in a common way. Similar to GRANT & REVOKE, so using a lot of those patterns with a similar test.

Supplementary improvements:

1. While I was working on that, I also noticed that EXEC was missing from the privileges list, so that's added now too (with a test example).
2. Additionally, SQL Server supports identifiers quoted [like this]. That dialect function hadn't been implemented yet, which caused a test failure on example that use that quoting style. So I've added that now too with a couple example cases for grant/deny.
3. Finally, "public" is a built in role on SQL Server, so doing something like grant select on mytable to public should parse public normally. However, the former logic was reading it as a keyword. This has been adjusted to be not special on SQL Server and tests cases have been added as well.