Turn useful libs into exts, add `ClimaCoreMakieExt`

The packages in `lib` were built years ago, before the notion of Julia extension existed.

Nowaways, these packages are largely forgotten and have received no meaningful update in years. As far as I can tell, the only two that see some use are `ClimaCoreSpectra` (used to compute spectra of Fields in ClimaAtmos) and `ClimaCorePlots` (used to plot the raw data associated to a Field).

I think it would be great to have something like `ClimaCorePlots`, but for `Makie` and as an extension. Ideally, I would like to be able to do:
```julia
import CairoMakie
heatmap(field)
```

Here is how Oceananingas does it: https://github.com/CliMA/Oceananigans.jl/blob/main/ext/OceananigansMakieExt.jl