feat: replace node pointer in place for much cheaper AST traversal

[jkumz]

This is a small addition that can make a significant dent in Go GC pressure under certain conditions. In something I'm building I was able to reduce my *Node allocations by 96%.

The standard pattern of calling cursor.Node() at each position allocates a new *Node wrapper. On certain batch jobs on very large codebases this can accumulate a lot of allocations.

It's additive so no compatibility issues with pre-existing implementations.

// Basic DFS traversal with node reuse
node := rootNode 
cursor := node.Walk()
defer cursor.Close()
current := cursor.Node() // allocate once
for {
    // Process current node
    doSomething(current)
    
    if cursor.GotoFirstChild() {
        cursor.UpdateNode(current) // update in place, no allocation
        continue
    }
    for !cursor.GotoNextSibling() {
        if !cursor.GotoParent() {
            return
        }
    }
    cursor.UpdateNode(current)
}```