HTML API: Refactor links_add_target() for more reliable parsing.

[dmsnell]

Trac ticket: Core-65984

This commit replaces the PCRE-based string operations used to find a given set of opening tags and then set their target attribute. It replaces it with the Tag Processor for clear, reliable, and direct updates.

It also deprecates the now-unused @access private function _links_add_target(), which was a helper function for the PCRE code and isn’t called anywhere else in Core or within the directory.

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