Shortcodes: Add shortcode processor.

[dmsnell]

Trac ticket: Core-66207.

This patch explores an efficient streaming interface for Shortcode processing. It’s currently only an exploration and not intended as a real proposal at this stage.

Synopsis

• Runs a forward pass to identify potential closing tags and eliminate algorithmic waste.
• Relies on WP_Token_Map when determining if a potential Shortcode tag ought to be recognized (based on which Shortcodes are registered).
• May stop processing at any point (after the candidate closing tags have been found).

Current limitations

• Searches without regard for HTML structure.
• Does not parse Shortcode attributes.
• Does not expose information about the matched Shortcodes.

[github-actions]

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