Add CREATE TRIGGER support for SQL Server

[aharpervc]

Adjacent to: #1808 with similar considerations, and temporarily rebased on it (eg, this branch should probably wait for that branch to merge).

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This PR introduces support for parsing CREATE TRIGGER for SQL Server.

The main concern is that for the existing dialects, there was an expectation of an EXECUTE keyword (PG: https://www.postgresql.org/docs/current/sql-createtrigger.html). However, SQL Server doesn't use this syntax and instead supports multi statement blocks (like a stored procedure).

The difficulty here in the codebase is what to do about CreateTrigger.exec_body & TriggerExecBody. In this iteration I made the property an Option, which seemed like the least impact on existing code. TriggerExecBody, etc can be left alone.

However in the future I think this could be improved with a more broad refactoring, such as a TriggerBody enum, which can consolidate TriggerExecBodyType's options & use Vec<Statements> for a MultiStatement variant and FunctionDesc for Function & Procedure variants. That'd basically remove the TriggerExecBody struct, too. Overall I think that'd be a cleaner approach for the CreateTrigger struct.

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That's all speculative followup work, for now I just want to be able to parse most SQL Server triggers. Other SQL Server specific things like particular trigger options, DDL trigger stuff, etc can come later as needed.

[aharpervc]

This fails to parse on this branch, but should:

CREATE TRIGGER some_trigger ON some_table FOR INSERT
AS
BEGIN
    IF 1=1
    BEGIN
        RAISERROR('Trigger fired', 10, 1);
    END

    RETURN;
END

Fixed, never mind

[aharpervc]

FYI for reviewers I rebased this on the create function branch, due to the return statement logic here: #1808 (comment)

[aharpervc]

Rebased again now that #1808 has been merged. Should be ready for review.