Fix pool pollution, infinite loop

[myndzi]

When nanoid is called with a fractional value, there were a number of undesirable effects:

• in browser and non-secure, the code infinite loops on while (size--)
• in node, the value of poolOffset becomes fractional, causing calls to nanoid to return zeroes until the pool is next filled: when i is initialized to poolOffset, pool[i] & 63 -> undefined & 63 -> 0
• if the first call in node is a fractional argument, the initial buffer allocation fails with an error

I chose |0 to cast to a signed integer primarily because that has a slightly better outcome in the third case above: if the first call is negative (e.g. nanoid(-1)) then Node will throw an error for an invalid Buffer size, rather than attempting to allocate a buffer of size 2**32-1. It's also more compact than >>>0, which would be necessary to cast to an unsigned integer. I don't think there is a use case for generating ids longer than 2**31-1 :)

The browser code is structured in such a way that casting size in customRandom succinctly isn't readily feasible. I chose to cast it at the line let j = step | 0 since casting defaultSize would not fix the infinite loop in all cases, and the other use of defaultSize is to define the step length which is already shown to be fractional and gets cast to an integer with ~ anyway.

As for the nanoid function, new Uint8Array(size) ignores the fractional part, and size doesn't get used further - the function instead calls reduce over the typed array.

In the Node/native async customAlphabet variant, I chose to convert the id.length === size check to id.length >= size, which handles the fractional case and avoids the infinite loop; size is not used for anything else there.