add {{model}} substitution macro to get name of current LLM

This is useful in the character card for an AI assistant, see #1774.

Tested with the Textgen backend, but should work with others too.

Horde will show only "Connected", and Novel will show the tier,
but Kobold and Textgen will show the model name.

If not connected, on any backend, will show "no_connection".

public/script.js

@@ -2179,6 +2179,7 @@ function substituteParams(content, _name1, _name2, _original, _group, _replaceCh
     environment.user = _name1 ?? name1;
     environment.char = _name2 ?? name2;
     environment.group = environment.charIfNotGroup = _group ?? name2;
+    environment.model = online_status;
 
     return evaluateMacros(content, environment);
 }