HTML API: Update Processing Instruction handling

[sirreal]

Processing instruction parsing has been updated in the HTML standard. <?processing instructions ?> were transformed into comments, but are now preserved.

See the processing instruction states in the standard.

See #12423 (merged here) for the test addition..

Trac ticket: https://core.trac.wordpress.org/ticket/65582

Use of AI Tools

AI assistance: Yes
Tool(s): Claude
Model(s): Fable 5
Used for: Initial implementation.

---

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

Test using WordPress Playground

The changes in this pull request can previewed and tested using a WordPress Playground instance.

WordPress Playground is an experimental project that creates a full WordPress instance entirely within the browser.

Some things to be aware of

• All changes will be lost when closing a tab with a Playground instance.
• All changes will be lost when refreshing the page.
• A fresh instance is created each time the link below is clicked.
• Every time this pull request is updated, a new ZIP file containing all changes is created. If changes are not reflected in the Playground instance,
  it's possible that the most recent build failed, or has not completed. Check the list of workflow runs to be sure.

For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation.

Test this pull request with WordPress Playground.

[sirreal]

We'll assume this is the intention. This entire insertion mode is gone from the current version of the spec: #9298

[sirreal]

This adds PI handling to all the relevant insertion modes. PI handling is notably missing from two places:

• in head noscript
• in template

This change currently follows the spec for in head noscript but inserts the token in template.

It's unclear whether these are an oversight or not. I've asked and will follow-up.