Type-unstable min/max

For unitless numbers, `min` and `max` promote their arguments to a common type. This does not happen for unitful quantities:
```julia
julia> max(1.0, 2)
2.0

julia> max(1.0u"m", 200u"cm")
200 cm
```
For type-stability and consistency with `Base`, I would prefer if `min`/`max` would promote their arguments for unitful quantities as well.

Edit: According to a comment in the tests, the current behavior is intentional:
https://github.com/PainterQubits/Unitful.jl/blob/09e761b8bb88cfdf7e252096385987f3c141f31e/test/runtests.jl#L476-L478

Start with the min/max tests in test/runtests.jl around lines 476-478 and read the comment explaining the intentional behavior. Compare the unitful behavior with Julia Base promotion, then determine with maintainers whether the intended result should change; done means an agreed behavior is reflected in the implementation and tests.

Written by the indexing model from the issue text.