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/project/src/project.rs

@@ -2280,11 +2280,13 @@ impl Project {
                 };
 
                 for (_, _, server) in self.language_servers_for_worktree(worktree_id) {
+                    let text = include_text(server.as_ref()).then(|| buffer.read(cx).text());
+
                     server
                         .notify::<lsp::notification::DidSaveTextDocument>(
                             lsp::DidSaveTextDocumentParams {
                                 text_document: text_document.clone(),
-                                text: None,
+                                text,
                             },
                         )
                         .log_err();
@@ -8325,3 +8327,19 @@ async fn wait_for_loading_buffer(
         receiver.next().await;
     }
 }
+
+fn include_text(server: &lsp::LanguageServer) -> bool {
+    server
+        .capabilities()
+        .text_document_sync
+        .as_ref()
+        .and_then(|sync| match sync {
+            lsp::TextDocumentSyncCapability::Kind(_) => None,
+            lsp::TextDocumentSyncCapability::Options(options) => options.save.as_ref(),
+        })
+        .and_then(|save_options| match save_options {
+            lsp::TextDocumentSyncSaveOptions::Supported(_) => None,
+            lsp::TextDocumentSyncSaveOptions::SaveOptions(options) => options.include_text,
+        })
+        .unwrap_or(false)
+}