Add extensions for TagHelperOutput to copy from TagBuilder.

- Tested out Merge, MergeAttributes, PullPrefixedAttributes and RestoreBoundHtmlAttribute.

#1319
nits:
- prefer `attributeName` since some earlier operation may have removed an entry from `tagHelperOutput.Attributes` (you don't know why the user is copying it)
- also update description to avoid narrow definition of the scenario