`convert_nodes_to_single_node` is inside its own file so ChildNode can
include and use it without having to include other headers such as
DOM/Node.h. This is to prevent circular includes.
This also moves getElementsByTagName to ParentNode to remove the code
duplication between Document and Element. This additionally fixes a bug
where getElementsByTagName did not check if the element was a
descendant, meaning it would also include the context element if the
condition matched.
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
