Add CREATE FUNCTION support for SQL Server

[aharpervc]

Partially related: #1800

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For reference: https://learn.microsoft.com/en-us/sql/t-sql/statements/create-function-transact-sql?view=sql-server-ver16

In SQL Server, functions look a lot like procedures, with begin/end, multiple statements, and a return. To accommodate that here in the parser, BeginEndStatement was extracted from the conditional logic to be reused here, along with a RETURN statement type

Details

Formerly, this was a new struct, but was later consolidated in with BeginEndStatement. I'm open to suggestions about how this could be improve (now or later). For example, since it's a BEGIN (statements) END construct, should this (and procedures) be using the statement block logic introduced here? https://github.com//pull/1791

Then also while I was working on this, I noticed that only SQL Server (so far...?) supports OR ALTER, so I introduced a new struct field to track that. (Again, very similar to CREATE PROCEDURE logic).

I'm not aiming to enable parsing every function type here in this PR -- just trying to get something operational, and the additional capabilities like table expressions, etc, can be added in later.

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To continue the recent discussion, I noticed that both these new multi statement functions & the existing procedure parsing fails unless the statements are semi-colon delimited. Semi-colons are optional in SQL Server, so for example, this code executes fine for real but fails to parse here in this library, unless that "SET" ends with a ;...

CREATE OR ALTER PROCEDURE test (@foo INT, @bar VARCHAR(256))
AS
BEGIN
    SET @foo = @foo + 1
    SELECT @foo
END

I think this is because the procedure body parsing logic is using parse_statements(), which requires it? I'm game to tackle the problem with some guidance on what's a good approach to make it parse 🤔