`unit` and `dimension` are inconsistent for `Level`s

For `Level`s (and mixed quantities based on `Level`s), `unit` discards the logarithmic part, `dimension` does not:
```julia
julia> unit(5u"dBV") # NoUnits

julia> dimension(5u"dBV") # voltage
𝐋^2 𝐌 𝐈^-1 𝐓^-3

julia> unit(5u"dBV*m") # meters (a length)
m

julia> dimension(5u"dBV*m") # voltage * length
𝐋^3 𝐌 𝐈^-1 𝐓^-3
```
I think the behavior of `dimension` makes sense. To fix `unit`, there are two possibilities:
1. Use the `unit` of the `reflevel`, i.e., `unit(5u"dBV") == V`
2. Make `unit(::Level)` throw an error

Edit: Related: #528.

Start by reproducing the documented `unit` and `dimension` examples for `Level`s and mixed quantities, then inspect the `Level` handling for both operations. Resolve whether `unit(::Level)` should use the reference-level unit or throw, and verify that the selected behavior is covered for both standalone and mixed quantities.

Written by the indexing model from the issue text.