Add Map.getImage()

[ZeLonewolf]

Fixes #2162

This PR adds Map.getImage(), which completes the CRUD methods for image access, along with Map.addImage(), Map.updateImage(), and Map.removeImage().

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.
• Write tests for all new functionality.
• Document any changes to public APIs.
• Add an entry to CHANGELOG.md under the ## main section.

[ZeLonewolf]

Going to add some more test cases

[ZeLonewolf]

This is good to go. I tried and failed to make the unit tests more compact, but at least it's testing all of the non-HTML image types.