Add DEFAULT as keyword

For (native) recursive CTE implementation, there is an optional CYCLE clause that includes the DEFAULT keyword:
CYCLE <exprs> SET <expr> TO <expr> DEFAULT <expr>

A potential issue with adding DEFAULT as keyword is that "default" is already used in other places without it being a keyword, e.g. as the default namespace name. Thus adding DEFAULT as keyword would normally require such uses to be quoted in backticks.

To minimize impact to existing users we will add DEFAULT as a special keyword that still allows the normal uses of "default" as (equivalent to) an identifier.