Interoperability with MeasureTheory

For use with MeasureTheory, it would be helpful if we could write `logfuncdensity(log_f, base)`, with expected behavior
```julia
logdensityof(logfuncdensity(log_f, base), x) == log_f(x) - logdensityof(base, x)
```

This would allow us to write
```julia
DensityInterface.logfuncdensity(log_f, base::Measure) = MeasureTheory.∫exp(log_f, base)
```

I know this package doesn't want base measures to be a requirement, so maybe we could find a way to make `base` optional.

`∫exp` is also defined for Distributions; it's not clear to me yet how to extend the interface to cover that case.