Add ast_helpers.raw_transaction

[fljdin]

Closes #7

• Completes the deparser.transaction_stmt() method to handle options for BEGIN or START TRANSACTION
• Adds a new helper to create a bunch of statements, inside a transaction block:

SELECT unnest(deparser.expressions_array(ast_helpers.raw_transaction(
  v_stmts := to_jsonb(ARRAY[
    ast.raw_stmt(
      v_stmt := ast.select_stmt(
        v_op := 'SETOP_NONE',
        v_targetList := to_jsonb(ARRAY[
          ast.res_target(v_val := ast.a_const(v_val := ast.integer(1)))
        ])
      ),
      v_stmt_len := 1
    )
  ]),
  v_isolation_level := 'REPEATABLE READ',
  v_deferrable := true
))) as transactions;

                   transactions                    
---------------------------------------------------
 BEGIN ISOLATION LEVEL REPEATABLE READ DEFERRABLE;
 SELECT 1;
 COMMIT;

[pyramation]

amazing work! thanks for including tests, this is totally awesome @fljdin :)