engineeringNotation sometimes has 10 decimal places

We don't truncate decimals on scaledMantissa, which sometimes leads to stupid JS floating point errors.

example failing spec:

it('correctly truncates floats', () => {
  expect(numberUtils.engineeringNotation(0.0000007)).toBe('700n');
});

// Expected: "700n"
// Received: "699.9999999999999n"