By default TypeScript includes all `@types/<name>` packages. This includes
@types/node, though we don't use it directly, because it is a transitive
dependency (see `yarn why @types/node`).

As a result Node's globals are added to the environment TS sees, even though
they don't really exist when our code runs in a browser. These globals
include overloads for `setTimeout` and `setInterval` which return a different
type (`Timeout`), causing spurious errors when assigning the result to a number.

Fix the problem by using the `types` option [1] in tsconfig.json to explicitly
specify which `@types/<name>` packages  to include when checking code in src/
This does mean that if we ever intentionally add @types packages which declare
globals (eg. for mocha), we'll need to explicitly list them here.

In the process the ES target was updated to fix an error about a `BigInt` reference.

[1] https://www.typescriptlang.org/tsconfig#types