- Provide a custom character range operation instead
- ensure that this correctly handles the astral plane, since those characters are two codepoints in JS (with UCS-2) rather than one (as in UTF-8)
- also need to make sure to get the *codepoint* of characters for determining the ranges, not the first/single byte value (which I think charcodeAt does?)
- Need a trackBranch wrapper for custom `either` implementations and such, to make the core aware of when branching is occurring - this is necessary for both reasonable errors ("expected X or Y or Z") as well as input management; in streaming modes, earlier input can only be discarded once it becomes unreachable for backtracking, which is only the case when there are no remaining levels of branching in the parsing tree
