fix(syntax): avoid quadratic allocation in Node::filter_map

[dotdash]

📌 What Does This PR Do?

filter_map_internal creates a new vector at every node, returning it and then extending the parent node' vector with it. This means a bunch of unnecessary allocations, and also O(n²) runtime behaviour due to the repeated copies from the child vectors into their parent vectors.

Passing the result vector down as an accumulator avoids both issues.

🔍 Context & Motivation

Making this go brrrrrrrr

🛠️ Summary of Changes

• Bug Fix: Avoid unnecessary allocations and copies.

📂 Affected Areas

• Linter
• Formatter
• CLI
• Dependencies
• Documentation
• Other (please specify):

🔗 Related Issues or PRs

none

📝 Notes for Reviewers

Summary
  ~/mago-quad --threads 32 lint --retain-code none ran
    1.05 ± 0.04 times faster than ~/mago-base --threads 32 lint --retain-code none