Refactor to only use untitled search editors and transparently convert to disk format on save/load. Ref #89268.

Ideally we'd fix by tracking if the match region was totally removed on every change operation, but that seems costly.

Fixes #89477.

Fixes #89466.

Properly implement onDidChangeContent (may be overridden by further refactorings, but simple enough so implementing here for now.) Fixes #89265.