HTML API: Add support for SVG and MathML (Foreign content) (remove file)

As part of work to add more spec support to the HTML API, this patch adds support for SVG and MathML elements, or more generally, "foreign content."

The rules in foreign content are a mix of XML and HTML parsing rules and introduce additional complexity into the processor, but is important in order to avoid getting lost when inside these elements.

This patch follows the first by deleting the empty files, which were mistakenly left in during the initial merge.

Developed in #6006
Discussed in https://core.trac.wordpress.org/ticket/61576

Follow-up to [58867].

Props: dmsnell, jonsurrell, westonruter.
See #61576.


git-svn-id: https://develop.svn.wordpress.org/trunk@58868 602fd350-edb4-49c9-b593-d223f7449a82

Author: dmsnell