GeezerVison

This PR changes the parser to account for some Snowflake-specific aspects of the MERGE statement, namely:

• Tolerance for the ON condition not being wrapped in parentheses
  • fixes Snowflake MERGE statement does not require parentheses around the ON statement #1315
  • fixes [FEATURE] MERGE ON without () #1880
• Support for an optional AND predicate after the MATCHED/NOT MATCHED part

See Snowflake's docs on MERGE: https://docs.snowflake.com/en/sql-reference/sql/merge - the new tests are derived from that doc.

With the ON logic, I did the simplest parser change which for existing cases (i.e. with parens), will now result in the onCondition being a Parenthesis containing the expression, where before it would have just been the expression (e.g. EqualsTo or whatever) itself. This feels innocuous to me, but if we consider this a breaking change and would prefer to avoid it, I can refactor to ensure full backwards compatibility, perhaps by adding a choice point in the parser and retaining the presence/absence of original parens as a boolean.