Addition of degree + radian is unitless

```julia
using Unitful
a = 180u"°" + 3.141u"rad"
@show unit(a) # blank
@show typeof(a) # Float64
```
I expect to have to explicitly ustrip() when I want a unitless quantity.
This behavior makes certain operations type-unstable, which the [docs](https://painterqubits.github.io/Unitful.jl/latest/trouble/#promotion-with-dimensionless-numbers) describe and dismiss.
Much of the point of having typed functions goes away if I also have to accept floats because their input units were automatically, silently canceled.
UnitfulAngles.jl does not fix this.

Start by running the reported Julia/Unitful example and read the linked documentation on promotion with dimensionless numbers. Done should prevent degree-plus-radian addition from silently producing a bare Float64, while preserving explicit conversion for callers who want a unitless value; the issue does not identify a file or test.

Written by the indexing model from the issue text.