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.

[Copilot]

Pull Request Overview

This PR updates the Tree-sitter dependency to v0.25.6 and adapts the Swift wrapper API to use opaque pointers and new encoding parameters.

• Upgraded the Tree-sitter Swift package version to 0.25.6.
• Replaced all UnsafePointer<TSLanguage> usage with OpaquePointer for language references.
• Updated TSInput initializer to include a new decode parameter and added explicit UTF-16LE/BE string encoding support.

Comments suppressed due to low confidence (1)

Sources/Runestone/TreeSitter/TreeSitterTextInput.swift:24

• Providing nil for the decode callback may break parsing of non-ASCII text under UTF-16LE/BE encodings because Tree-sitter relies on a decode function to convert bytes to codepoints. Consider supplying a proper decode callback (as in swift-tree-sitter) or verifying that default behavior works for all target encodings.

        return TSInput(payload: payload, read: read, encoding: encoding, decode: nil)

[Copilot]

The original TSInputEncodingUTF16 case was removed from stringEncoding, so any code still using TSInputEncodingUTF16 will fall through and return nil, causing silent parse failures. Consider explicitly handling or deprecating UTF16, or adding a mapping for it.