Add grammar support for insert...returning... (most cases)

[angelamayxie]

Part of dolthub/dolt#9248

Insert...returning... statements work with insert_data_alias and SET assignment_list cases. The following examples (from the MariaDB documentation) work.

INSERT INTO t2 (id) VALUES (2),(3) RETURNING id,t;

INSERT INTO t1 SET id1=1, animal1='Bear' RETURNING f(id1), UPPER(animal1);

I wasn't able to get insert...returning... to work with insert_data_select cases. The RETURNING clause is not supported for the following and would lead to a syntax error.

INSERT INTO archive (id, data) SELECT id, data FROM live_data WHERE flag = 'old' RETURNING id;

When I had added a RETURNING clause, there was a grammar conflict because the parser was not able to distinguish between a column alias and the 'RETURNING' keyword. This could be resolved by making RETURNING a reserved keyword, but we decided against that since RETURNING is a non-reserved keyword in MySQL (doc).

I ran into a similar conflict with insert_data_alias cases because the parser was not able to distinguish between a table alias and the RETURNING keyword. This was resolved by adding RETURNING to non_reserved_keyword2. This means RETURNING can't be used as a table alias and may cause some customer issues but the likelihood is very low.

Also removed TestSingle from parse_test.go (we already have TestSingleSQL)

[fulghum]

Looks good! Nice work on this one.