Fixes #4855

Update the document selection logic in lit-html.ts to also check whether
document.createTreeWalker is a function:

const d =
  NODE_MODE && typeof global.document?.createTreeWalker !== 'function'
    ? ({ createTreeWalker() { return {}; } } as unknown as Document)
    : document;

If the function is missing, lit-html falls back to its internal mock.

Did not add a unit test here, since the fallback path is decided at module evaluation time (const d = ...), and in the existing test files lit-html is already imported at the top.
This makes it impossible to toggle document.createTreeWalker before the module loads without introducing brittle dynamic-import tricks or extracting a new helper function, which I wanted to avoid to keep this fix minimal.