Support IF NOT EXISTS for CREATE SCHEMA (#276)

This is a Postgres-specific clause: https://www.postgresql.org/docs/12/sql-createschema.html

Also add a test for `DROP SCHEMA IF EXISTS schema_name`, which is already supported in the parser.

src/parser.rs

@@ -1078,8 +1078,12 @@ impl<'a> Parser<'a> {
     }
 
     pub fn parse_create_schema(&mut self) -> Result<Statement, ParserError> {
+        let if_not_exists = self.parse_keywords(&[Keyword::IF, Keyword::NOT, Keyword::EXISTS]);
         let schema_name = self.parse_object_name()?;
-        Ok(Statement::CreateSchema { schema_name })
+        Ok(Statement::CreateSchema {
+            schema_name,
+            if_not_exists,
+        })
     }
 
     pub fn parse_create_external_table(