HTML API: Prevent incorrect newline injection in non-html TEXTAREA, etc.

[sirreal]

HTML PRE, LISTING, and TEXTAREA elements ignore a leading newline. Previous changes ensured those newline were preserved on normalization (r61747), but did not account for non-HTML examples of these elements (for example svg:textarea).

Ensure the newlines are not incorrectly added to analogous non-HTML elements. Currently, successive normalizations of the following produce:

1. <svg><textarea>X
2. <svg><textarea>\nX</textarea></svg>
3. <svg><textarea>\n\nX</textarea></svg>
4. etc…

Trac ticket: https://core.trac.wordpress.org/ticket/65372
Follow-up to r61747

Use of AI Tools

AI assistance: Yes
Tool(s): Codex GPT-5.5
Used for: Fuzz testing, diagnosis, initial fix implementation, review and feedback.

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