add outputOffset to the `into` methods

[bakkot]

Split out out from https://github.com/tc39/proposal-arraybuffer-base64/pull/33.

Won't be in this proposal unless implementation or user feedback demonstrates need during stage 3.

[syg]

Some numbers, finally. From this: https://jsperf.app/qafule/12

Looks like about the following speedups from using an offset:

• Chrome: 28% faster
• Safari: 18% faster
• Firefox: 31% faster

Microbenchmark, but those numbers mean this decision can affect performance on the hot path.

[jridgewell]

I made another micro benchmark (one using TextEncoder.encodeInto()) that shows an even more stark improvement.

• Using a write-offset and str.substring() is:
  • Chrome: 53% faster
  • Firefox: 138% faster
  • Safari: 71% faster
• Using a write-offset and read-offset:
  • Chrome: 16% faster (?)
  • Firefox: 158% faster
  • Safari: 122% faster

For fun, I also threw in an attempt to use premade subarrays to use with TextEncoder.encodeInto. Doing so is also considerably faster than creating subarrays on-the-fly:

• Chrome: 37% faster
• Firefox: 49% faster
• Safari: 29% faster

I don't know if it's expensive to create subarrays, or if it's the GC pressure, but subarrays suck.