Investigate NODE-4401 - [js-bson] enable "isolatedModules" typescript option

NODE-4401 Description
I'd would be great if `js-bson` could be compiled with `isolatedModules` set to `true`

See https://www.typescriptlang.org/tsconfig#isolatedModules for details on this option.

Why?

Im working on an automatically ported, feature complete version of `js-bson` for deno - see:

 * https://github.com/thekorn/bson-deno

However deno requires isolatedModules to be set to true. My goal is to port `js-bson` in a minimal inversive fashion, so any required change which is already done upstream would help alot