chore: Add node10 entrypoint to Foundation

[spalladino]

Using node10 module resolution fails when importing Foundation, since the exports entry with multiple entrypoints is ignored. This PR adds a main fallback with an index that re-exports every submodule as a named object.

So a user with node10 module resolution and targetting a commonjs module can do:

import { sleep } from '@aztec/foundation';

async function main() {
  await sleep.sleep(1);
}

While a user targetting node16 and up can do:

import { sleep } from '@aztec/foundation/sleep';

async function main() {
  await sleep(1);
}

Note that importing @aztec/foundation under node16 fails, since there is no . entrypoint under exports.