Add support for the experimental Next LS for Elixir (#3024)

This is a PR I built for a friend of a friend at StrangeLoop, who is
making a much better LSP for elixir that elixir folks want to experiment
with. This PR also improves the our debug log viewer to handle LSP
restarts.

TODO:
- [ ] Make sure NextLS binary loading works.

Release Notes:

- Added support for the experimental Next LS for Elxir, to enable it add
the following field to your settings to enable:

```json
"elixir": {
    "next": "on"
}
```

crates/zed/src/zed.rs

@@ -2388,11 +2388,12 @@ mod tests {
 
     #[gpui::test]
     fn test_bundled_languages(cx: &mut AppContext) {
+        cx.set_global(SettingsStore::test(cx));
         let mut languages = LanguageRegistry::test();
         languages.set_executor(cx.background().clone());
         let languages = Arc::new(languages);
         let node_runtime = node_runtime::FakeNodeRuntime::new();
-        languages::init(languages.clone(), node_runtime);
+        languages::init(languages.clone(), node_runtime, cx);
         for name in languages.language_names() {
             languages.language_for_name(&name);
         }