feat: restrict sideEffects to CSS files in package.json

[CommanderStorm]

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!

We are missing the sideEffects flag in our package-json, which means that bundlers currently have to be extra cautios when optimising us.
The *.css is something that we can likely also remove, but this is a bit unclear to me if they are actually side effect free
This is uneesseary and we can instead just let them optimise us.

I am putting this into v6 since it is fairly hard to predict if this is actually safe.
According to our tests it is, and we are not using side-effect ridden things in the main apis that I looked at (plus in the many more minor APIs that I let claude audit).

[HarelM]

I need to read about what this means, never saw it before. Thanks for taking the time to open this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.71%. Comparing base (f293c20) to head (55abb48).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7258   +/-   ##
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  Coverage   92.71%   92.71%           
=======================================
  Files         289      289           
  Lines       24074    24074           
  Branches     5112     5112           
=======================================
  Hits        22321    22321           
  Misses       1753     1753           

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