Improve hillshade layers not using raster-dem sources error message

[ibesora]

This PR improves the errors around adding hillshade layers that don't use a raster-dem source.

Before this PR, adding a hillshade layer with a raster source said layer \"test\" requires a vector source" which is not right.
With this PR, the same use case results in the following error: layer \"test\" requires a raster-dem source.
It also adds a couple of tests to check that and an extra one that tests a condition that was not met before (using a raster-dem source type on layers that are not hillshade)

Launch Checklist

• Confirm your changes do not include backports from Mapbox projects (unless with compliant license) - if you are not sure about this, please ask!
• Briefly describe the changes in this PR.
• Link to related issues.
• Include before/after visuals or gifs if this PR includes visual changes.
• Write tests for all new functionality.
• Document any changes to public APIs.
• Post benchmark scores.
• Add an entry to CHANGELOG.md under the ## main section.

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (58eb347) 77.99% compared to head (58c417d) 78.02%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #353      +/-   ##
==========================================
+ Coverage   77.99%   78.02%   +0.03%     
==========================================
  Files         101      101              
  Lines        4140     4142       +2     
  Branches     1183     1184       +1     
==========================================
+ Hits         3229     3232       +3     
+ Misses        911      910       -1     

Files	Coverage Δ
src/validate/validate_layer.ts	100.00% <100.00%> (+1.47%)	⬆️

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[HarelM]

Let's add this to 19.3.2 as well...