debugger: Fix the JavaScript debug terminal scenario (#33924)

There were a couple of things preventing this from working:

- our hack to stop the node REPL from appearing broke in recent versions
of the JS DAP that started passing `--experimental-network-inspection`
by default
- we had lost the ability to create a debug terminal without specifying
a program

This PR fixes those issues. We also fixed environment variables from the
**runInTerminal** request not getting passed to the spawned program.

Release Notes:

- Debugger: Fix RunInTerminal not working for JavaScript debugger.

---------

Co-authored-by: Cole Miller <cole@zed.dev>

crates/proto/proto/debugger.proto

@@ -535,7 +535,7 @@ message DebugScenario {
 
 message SpawnInTerminal {
     string label = 1;
-    string command = 2;
+    optional string command = 2;
     repeated string args = 3;
     map<string, string> env = 4;
     optional string cwd = 5;