Bump `TreeSitter` version to 0.25.6

[ewilken]

Here we go again, as a continuation of #401. 🙂

TSLanguage doesn't seem to be publicly exported anymore on the latest version of tree-sitter (or its Swift package wrapper), so I've used OpaquePointers, inspired by swift-tree-sitter's implementation. Eyeballing it, it didn't break anything for me, but please let me know what you think of this.