v.timeRangeStart isn't a consistent type

Steps to reproduce:
This is a comment from a user in the Flux slack. It is point number 4 in https://github.com/influxdata/flux/issues/3507.

In summary, the v.timeRangeStart variable isn't a consistent type which makes it more convoluted to perform duration math on it. Sometimes it is a time and sometimes it is a duration for a relative time. We should consider just choosing one type and going with that. Because it is not a consistent type, it makes processing that time a bit more difficult. While range() can accept both times and durations, other methods cannot necessarily do that so either time(v:) or duration(v:) are needed.

I believe the Flux language is working on making it so more of the time functions will accept either and behave the same, but I think it's still useful to discuss the possibility of setting this specific variable to a single type.

Expected behavior:
When creating a flux script, the v.timeRangeStart variable should be a consistent type.

Actual behavior:
It is either a time or duration type depending on the input.

Start with the referenced Flux issue #3507 and inspect how v.timeRangeStart is produced for absolute and relative ranges, including how range() handles each type. A complete change would choose and document one consistent type for v.timeRangeStart and verify that duration and time operations behave accordingly.