Add outputhandler to a plugin definition

e.g.:
%%js
function processLispOutput(r) {
if(r.val) return r.val;
return Utils.formatJson(r);
}

%% plugin
{
"languageId": "lisp",
"displayName": "Microtalk Lisp",
"url": "https://ds604.neocities.org/js/microtalk.js",
"module": "MICROTALK",
"evaluator": "evaluate",
"pluginType": "language",
"outputHandler":"processLispOutput"
}

Start by locating the plugin-definition parsing and language evaluation entry points, then trace how evaluator results are formatted. Add support for the outputHandler name shown in the example and verify that its return value controls displayed output; the issue names no specific files or tests.

Written by the indexing model from the issue text.