Compute scrollbar markers asynchronously (#10080)

Refs #9647
Fixes https://github.com/zed-industries/zed/issues/9792

This pull request moves the computation of scrollbar markers off the
main thread, to prevent them from grinding the editor to a halt when we
have a lot of them (e.g., when there are lots of search results on a
large file). With these changes we also avoid generating multiple quads
for adjacent markers, thus fixing an issue where we stop drawing other
primitives because we've drawn too many quads in the scrollbar.

Release Notes:

- Improved editor performance when displaying lots of search results,
diagnostics, or symbol highlights in the scrollbar
([#9792](https://github.com/zed-industries/zed/issues/9792)).

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Nathan <nathan@zed.dev>

crates/workspace/Cargo.toml

@@ -25,6 +25,7 @@ test-support = [
 
 [dependencies]
 anyhow.workspace = true
+any_vec.workspace = true
 async-recursion.workspace = true
 bincode = "1.2.1"
 call.workspace = true