Add Keyword::Default for SimpleExpr

[tisonkun]

This is useful when one would like to make an insert clause with partially values set as DEFAULT, like:

INSERT INTO "my_table" ("my_col_0", "my_col_1") VALUES (1, DEFAULT), (DEFAULT, 42)

Currently, I use Expr::cust("DEFAULT") to work it around. But I wonder if a Keyword::Default desired.