perf: array optimizations

[GalacticHypernova]

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

While a less-known optimization, pre-allocating arrays allows the JavaScript engine to prepare all the memory upfront for the array and prevent dynamic resizing. Directly accessing arrays would be O(1) as opposed to Array.prototype.push being amortized O(1) due to the occasional dynamic resizing which is O(n).

This PR, while having the pre-allocated arrays as the core change, simplifies affected areas by, for example, reducing iteration and mapping counts for arrays whose size can be determined immediately, and thus can be pre-allocated. By doing that, in areas with multiple arrays (such as the CSS plugin that registers the initial root variables), we can use a single loop and consolidate the mapping logic directly in it.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[GalacticHypernova]

Marking as draft so I could take my time and make sure I get everything I can

[benjamincanac]

What are you trying to solve here? 🤔