API: Add banned_strings

From exllamav2: List of strings that the generator will refuse to output. As soon as a partial match happens, a checkpoint is saved that the generator can rewind to if need be. Subsequent tokens are then held until the full string is resolved (match or no match) and either emitted or discarded, accordingly.

sampler_overrides/sample_preset.yml

@@ -18,6 +18,10 @@ stop:
   override: []
   force: false
   additive: false
+banned_strings:
+  override: []
+  force: false
+  additive: false
 token_healing:
   override: false
   force: false