HTML API: Replace Open Elements handlers with explicit stack accounting

[dmsnell]

Trac ticket: Core-65383

The push and pop handlers in the HTML API were convenient when initially building the class, but being indirect, they involve an overhead when calling and require additional protection for avoiding calling the wrong handlers when and if an instance of the class were to be unserialized.

This patch replaces the direct calls by modifying the return values of the methods in the Open Elements calss to return pushed or popped elements. This makes it possible for the HTML Processor to track its own accounting directly while still delegating the stack operations to the Open Elements.

Patch developed by Codex on instruction from dmsnell.

[github-actions]

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