Optimize syntactic classification walk

[CyrusNajmabadi]

Drops us about 0.7% during scrolling from:

to

[ToddGrun]

if (child.IsNode)

nit question: I've seen this in a couple places, why not just use the "bool AsNode(out SyntaxNode? node)" method?

[sharwell]

If possible, I would prefer it be called IsNode(out SyntaxNode?). I don't think this is possible with a property and method in the same type, but may be possible with it defined as an extension method.

[CyrusNajmabadi]

I didn't know the AsNode method existed. Will update to that.

[CyrusNajmabadi]

done.

[ToddGrun]

if (childNode.FullSpan.IntersectsWith(_textSpan))

if this is a hot path, maybe breaking out of the inner loop is worthwhile:

if (childNode.FullSpan.IntersectsWith(_textSpan))
    stack.Push(childNode);
else if (childNode.Position > _textSpan.End)
    break;

[CyrusNajmabadi]

I'll def try that

[CyrusNajmabadi]

done

[ToddGrun]

ChildNodesAndTokens

this is way off topic, but I took another look at the SyntaxList.Enumerator's call to ItemInternal, and I'm not clear why the slotIndex isn't held between calls. By not doing so, the first loop in ItemInternal would need to walk the green slots until it reaches the appropriate slot index.

Again, off topic, so nothing needed in this PR, but it seems odd.

[CyrusNajmabadi]

def look into that. there are some comments explaining things. but it's possible they're stale, or there is room for improvement here.

[CyrusNajmabadi]

Note: i'm saying @ToddGrun should look into this :)

[ToddGrun]