Let the extension use the Dafny server to track changes in dfyconfig.… (

#435)

### Changes
- Let the extension use the Dafny server to track changes in
`dfyconfig.toml` files

### Testing
- Manually assert that changes in `dfyconfig.toml` files are picked up
instantly, without saving the file.
- Assert that only opening the `dfyconfig.toml` already publishes
diagnotics

package.json

@@ -123,7 +123,8 @@
         ],
         "extensions": [
           ".dfy",
-          ".dfyi"
+          ".dfyi",
+          "dfyconfig.toml"
         ],
         "configuration": "./language-configuration.json"
       }