Implement DML RETURNING clause

[johnedquinn]

Relevant Issue/Bug

• Ambiguous Returning Clause for INSERT #708
• Adds DML (INSERT, DELETE, UPDATE, UPSERT, REPLACE) #1666

Requested Solution/Feature

• In Adds DML (INSERT, DELETE, UPDATE, UPSERT, REPLACE) #1666, the returning clause in DML was removed intentionally, though, it will likely need to be re-added in the future.

Additional Context

• This can be seen in DDB documentation
• Also in PostgreSQL docs
• However, the grammar rules for this were poor. This should be an optional field of the Insert and Update statement AST nodes to more directly represent its placement in the syntax. Rather than being added to ALL DML statements.

DoD (Definition of Done)

• Proper grammar rules
• Proper AST modeling
• Correct parsing