Previously, the `s` argument of the `WP_Query::parse_query()` method searched the `post_title`, `post_excerpt`, and `post_content` fields, with no way of controlling this apart from using the `posts_search` filter and adjusting the SQL manually. This changeset adds the ability to specify which fields are searched when performing a query, using the `search_columns` argument.
Props johnbillion, birgire, petitphp, audrasjb, costdev, mukesh27.
Fixes#43867.
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