Improve default matching behavior for VIP Enterprise Search

Originally reported by @scottnelle in an internal Alley repo:

> VIP Enterprise search seems to default to phrase matching which requires that all terms in the query (or maybe even the entire exact phrase) appears in a field for an article to match. This makes longer search phrases very unlikely to match any articles. For example, on a site that is almost entirely about home improvement, the phrase "how to paint my house" yields zero results because the writing is rarely in first person.

> We should apply a more flexible default query type, perhaps inspired by the defaults set in SearchPress https://github.com/alleyinteractive/searchpress/blob/d7feab20ee2214caa574e410d535414978c00182/lib/class-sp-wp-search.php#L250-L251