add test for BigInt as Map key?

[dckc]

I suggest a test for Map([[1n, "abc"]]).has(1n) and the like, analagous to https://github.com/tc39/test262/blob/main/test/built-ins/Map/symbol-as-entry-key.js

cc @erights

ref https://github.com/Moddable-OpenSource/moddable/issues/577

[rwaldron]

Nice! Let's do it