Serialize projectionDefinition to arrays

[birkskyum]

This is how it would look to serialize to arrays like color does.

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.
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• Add an entry to CHANGELOG.md under the ## main section.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.78%. Comparing base (e56dada) to head (d921832).

Additional details and impacted files

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##           projection-expression-collecting-branch     #906   +/-   ##
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  Coverage                                    92.77%   92.78%           
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  Files                                          107      107           
  Lines                                         4736     4741    +5     
  Branches                                      1345     1345           
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+ Hits                                          4394     4399    +5     
  Misses                                         342      342           

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[HarelM]

I think interpolate should return the same type of object and not a string...

[birkskyum]

Okay,

Just to clarify, this does return an array, not a string.

[HarelM]

I'm failing to see the motivation behind this PR to be honest...

[birkskyum]

@HarelM , thanks for being honest with it, I'm fine with either.

I do see that color does get serialized to array somehow (saw it in test/integration/expression/tests/interpolate-hcl/linear/test.json test), but I couldn't figure out where it happens.

My main motivation here was the comment above. I wanted to ensure that we made our choice on API design based, if we serialize to arrays or class objects, based on preference, instead of having technical limitations make the decision for us.