HTML API: Fix extensibility of `WP_HTML_Processor::next_token()`. #36192

Job	Run time
Check PHP compatibility / Run compatibility checks	17s
Slack Notifications / Prepare notifications	2s
Slack Notifications / Success notifications	2s
Slack Notifications / Failure notifications	0s
Slack Notifications / Fixed notifications	0s
Slack Notifications / Cancelled notifications	0s
Failed workflow tasks	0s
	21s

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