Suggestion: Human-readable unit conversion

(Just throwing an idea, sorry if this is a duplicate — I could not find an issue with it)

Maybe a function (called `human`?) could be added to help with reading some units? I sometimes prefer to read numbers with a "human-readable" format and unit. E.g., I prefer to read `1.23 μs` rather than `1.234567898765432e-6 s`. Here two things were done, (i) rounding to show only 3 digits, which I think is good enough in general, and (ii) conversion to microseconds to make sure the answer is in the small, readable, half–open interval [1,1000).
So that one would do something like:
```julia
julia> x = 0.000001234567898765432u"s"
1.234567898765432e-6 s

julia> human(x)
1.23 μs

julia> x |> human
1.23 μs
```
Maybe it could use some `digits` keyword argument too? I.e.,
```julia
julia> human(x, digits=2)
1.2 μs
```
Again, just throwing an idea out there. Don't hesitate to close if this is a dumb feature request! 😅

No file, test, or entry point is identified in the issue. Begin by locating Unitful.jl’s existing quantity display and unit-conversion code, then settle the rounding, preferred-unit range, and digits semantics; the feature is done when the shown examples are covered by tests.

Written by the indexing model from the issue text.