python: Uplift basedpyright support into core (#35250)

This PR adds a built-in adapter for the basedpyright language server.

For now, it's behind the `basedpyright` feature flag, and needs to be
requested explicitly like this for staff:

```
  "languages": {
    "Python": {
      "language_servers": ["basedpyright", "!pylsp", "!pyright"]
    }
  }
```

(After uninstalling the basedpyright extension.)

Release Notes:

- N/A

crates/languages/Cargo.toml

@@ -41,6 +41,7 @@ async-trait.workspace = true
 chrono.workspace = true
 collections.workspace = true
 dap.workspace = true
+feature_flags.workspace = true
 futures.workspace = true
 gpui.workspace = true
 http_client.workspace = true