DOM: Verify Node.lookupPrefix works according to the spec

[karfau]

As part of #321 a docstring was added to the implementation of Node.lookupPrefix including links to the expected implementation.

• There seems to be some missing cases for some node types when comparing it to the specs
• We need to test that it also works as expected when the is DOM is created using the DOMImplementaiton methods or manipulated after parsing.
• We should understand if there is a difference in the specification between DOM Level 3 and the living standard
• We need to either document the differences or implement it according to the spec
  • living standard should win over DOM Level 3 in case of conflicts
  • this might be a breaking change
  • Maybe it's possible to get rid of the internal _nsMap (we should compare performance though, since this method is used as part of serialization)