core.ts does not test whether BigInt exists

https://github.com/benjamn/ast-types/blob/master/def/core.ts#L338 assumes BigInt to exist, but it doesn't in the environment where my code runs (Zotero, which is technically built on firefox ESR), so my code errors out.

Start with def/core.ts at line 338 and inspect how BigInt is referenced during core definitions. Reproduce the failure in an environment where BigInt is unavailable, such as the reported Firefox ESR-based Zotero environment, then verify that the core definitions load without that error when BigInt is absent.

Written by the indexing model from the issue text.