debugger: Fix running JS tests when worktree root and package root do not coincide (#32644)

- construct the correct path to the test library based on the location
of package.json
- run scripts from the package root where they were defined
- run tests in the directory of the defining file

Release Notes:

- Debugger Beta: fixed running JS tests when the worktree root is above
the location of package.json.

---------

Co-authored-by: Anthony <anthony@zed.dev>
Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/project/src/toolchain_store.rs

@@ -278,7 +278,7 @@ impl language::LanguageToolchainStore for RemoteStore {
     }
 }
 
-pub(crate) struct EmptyToolchainStore;
+pub struct EmptyToolchainStore;
 #[async_trait(?Send)]
 impl language::LanguageToolchainStore for EmptyToolchainStore {
     async fn active_toolchain(