feat(expr): support function date

[xiangjinwu]

Is your feature request related to a problem? Please describe.

select date('2020-01-23');

Used by looker generated queries.
-- @jetjinser

Describe the solution you'd like

This is essentially the cast operation, but written as a function call. We can just bind this function to the existing operation.

Describe alternatives you've considered

In PostgreSQL, all proname from pg_proc are exposed as user-callable, which include internal names like int2 for casting to smallint and int48gt for int > bigint. We do not intend to support all such function names all together, unless required to integrate with an existing PostgreSQL tool.

Additional context

No response