Add dust map extension for SkyCoords

[cgarling]

Adds SkyCoordsExt which allows querying SFD98Map with SkyCoords.AbstractSkyCoords via method(m::SFD98Map)(s::SkyCoords.AbstractSkyCoord). I verified tests pass locally but I'm not able to build the docs locally (they stall out and do not finish) so idk if the docs are set up properly or not yet.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (e4b3586) to head (932fce2).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #85   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           11        12    +1     
  Lines          347       351    +4     
=========================================
+ Hits           347       351    +4     

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[cgarling]

Docs are building, apparently just takes longer than I expected
https://juliaastro.org/DustExtinction.jl/previews/PR85/dust_maps/