Formatting: Add excerpt_allowed_tags filter to preserve HTML tags in wp_trim_excerpt()

[himanshupathak95]

Trac ticket: https://core.trac.wordpress.org/ticket/12084

wp_trim_excerpt() has always stripped all HTML from auto-generated excerpts unconditionally, via wp_trim_words() → wp_strip_all_tags(). There was no hook to preserve any markup, making it impossible for themes and plugins to retain semantic HTML (e.g., <strong>, <em>, <a>) in excerpts without replacing the entire wp_trim_excerpt function.

This PR introduces a new excerpt_allowed_tags filter in wp_trim_excerpt() that defaults to an empty string (fully backward compat). When a non-empty allowed-tags string is supplied, a new HTML-aware trimming path is
taken instead of the wp_trim_words() path.

Usage

add_filter( 'excerpt_allowed_tags', function () {
    return '<strong><em><a>';
} );

Testing

You can run and check the tests using -

npm run test:php -- --filter=Tests_Formatting_wpTrimExcerpt 

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