HTML API: Add PLAINTEXT tag handling

[sirreal]

A start tag whose tag name is "plaintext"
If the stack of open elements has a p element in button scope, then close a p element.

Insert an HTML element for the token.

Switch the tokenizer to the PLAINTEXT state.

Once a start tag with the tag name "plaintext" has been seen, that will be the last token ever seen other than character tokens (and the end-of-file token), because there is no way to switch out of the PLAINTEXT state.

I started work on <plaintext> tags, but they don't make much sense until we can support text nodes (#5683). Pausing this work for now.

Trac ticket: https://core.trac.wordpress.org/ticket/60283

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[dmsnell]

I think we should actually change the Tag Processor before doing this, because we will continue to step into new tags if we don't.