Introduce an internal version of replaceChildren

[skirtles-code]

replaceChildren() is used by addProps() and betweenChildren() internally. This PR introduces replaceChildrenInternal() that all 3 can use instead.

replaceChildrenInternal() doesn't have the dev warnings for the arguments. This avoids duplicate warnings when addProps() or betweenChildren() call it, or when it calls itself recursively. This does potentially remove an argument check on the children for recursive calls, but that really isn't an edge case those checks were intended to catch. The warnings in that unlikely scenario would have been confusing anyway.

replaceChildrenInternal() also doesn't specify a default for the options argument. This potentially allows SKIP_COMMENTS to be tree-shaken if replaceChildren() isn't used directly.