Refactor: Rewrite convert_smilies to avoid catastrophic runtime performance.

[dmsnell]

Todo

• Find catastrophic case and demonstrate it not crashing.
• Benchmark typical performance.
• Perform string replacement in the HTML API without having to decode? This would help make the existing tests pass and could minimize the string changes that need to be applied to a document.
• Respond to changes in $wpsmiliestrans - it currently assumes that filtering of that list only occurs once at bootup, but some code directly modifies the global.
• Separate wp_find_whitespace_span() into separate patch?

Summary

In some cases, the search for smilies to replace can introduce catastrophic backtracking in the PCRE pattern built to find them.

In this patch the function is rewritten using the HTML API and the WP_Token_Map, and all PCRE methods have been replaced with hand-written single-pass parsers where applicable.

The new approach should avoid memory allocation and catastrophic runtimes and crashes.

[github-actions]

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