feat: Implement the TreeWalker API

[Hunam6]

Ref: https://developer.mozilla.org/en-US/docs/Web/API/TreeWalker

[b-fuze]

Yeah, it's on the list of things to implement when I get some time.

[Danny-Engelman]

@b-fuze , I presume you also didn't do https://developer.mozilla.org/en-US/docs/Web/API/NodeIterator ?

TreeWalker is the faster way to walk the DOM, NodeIterator only slightly slower

Traversing is SLOW

https://www.measurethat.net/Benchmarks/Show/5581/2/traverse-function-vs-nodeiterator-vs-treewalker#latest_results_block

[b-fuze]

@Danny-Engelman no, I didn't do either of them. But TreeWalker is on the plan to to do list.

[0xAozora]

#37

[chances]

This issue is blocking my integration of lit-html in a project.