Create nodes in the realm of the document they are created for

Which realm a node is created in was unspecified, so implementations
disagreed when a node was created for a document whose relevant realm
differed from the one running the algorithm (e.g. parsing, importing, or
cloning into an adopted node's document).

This also makes the HTML parser initialize processing instructions,
populating their attribute map as createProcessingInstruction() and the
constructor already did.

Fixes #977.

Tests: web-platform-tests/wpt#61212

Author: annevk