Remove unnecessary temporary array allocation

[Sergio0694]

Overview

This PR adds an API to enumerate items in an array builder without allocating a whole array.
It also uses this API when creating a BlockSyntax, to avoid wasting an array just to enumerate items.
The returned enumerable is readonly, ensuring mutations must go through the actual builder.

PR Checklist

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• PR doesn't include merge commits (always rebase on top of our main, if needed)
• Tested code with current supported SDKs
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes
• Every new API (including internal ones) has full XML docs
• Code follows all style conventions

[Sergio0694]

Hey @Youssef1313, want to take a quick lock when you have a minute? This one is very smol 😄

[Sergio0694]

@Youssef1313 I assume you have no other concerns with this one, so good to go? 😄

[Youssef1313]

Yup. Looks good!