Search: Option to expand all results by default

[JacksonKearl]

Currently, in the search viewlet, we automatically collapse results from files with more than 10 hits.

This came from a time when adding many tree entries was more expensive; now that we can afford to show more items, it would be nice to have the auto-collapse logic as a configureable setting. We could make results either never collapse, or allow users to configure the collapse threshold away from 10.

Similar to #48842. Related to #28090.

[JacksonKearl]

Turns out we already had this: "search.collapseResults": "alwaysExpand"