search: New old search implementation (#39956)

This is an in-progress work on changing how task scheduler affects
performance of project search. Instead of relying on tasks being
executed at a discretion of the task scheduler, we want to experiment
with having a set of "agents" that prioritize driving in-progress
project search matches to completion over pushing the whole thing to
completion. This should hopefully significantly improve throughput &
latency of project search.

Release Notes:

- Improved project search performance

---------

Co-authored-by: Smit Barmase <smit@zed.dev>
Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>

Cargo.toml

@@ -126,6 +126,7 @@ members = [
     "crates/picker",
     "crates/prettier",
     "crates/project",
+    "crates/project_benchmarks",
     "crates/project_panel",
     "crates/project_symbols",
     "crates/prompt_store",
@@ -194,6 +195,7 @@ members = [
     "crates/web_search_providers",
     "crates/workspace",
     "crates/worktree",
+    "crates/worktree_benchmarks",
     "crates/x_ai",
     "crates/zed",
     "crates/zed_actions",