Add support for DreamGen API.

API docs: https://dreamgen.com/docs/models/opus/v1
API keys: https://dreamgen.com/account/api-keys

I decided to base this on the text-completion API since it's more
flexible with SillyTavern's prompt formating capabilities.

This also means that custom context and instruct settings are required.

Will add documentation in a followup PR.

public/scripts/preset-manager.js

@@ -315,6 +315,7 @@ class PresetManager {
             'custom_model',
             'bypass_status_check',
             'infermaticai_model',
+            'dreamgen_model',
             'openrouter_model',
             'max_tokens_second',
         ];