Method addPlugin

packages/less/lib/less/plugin-manager.js:34–42

* * @param plugin * @param {String} filename

(plugin, filename, functionRegistry)

     * @param {String} filename
     */
    addPlugin(plugin, filename, functionRegistry) {
        this.installedPlugins.push(plugin);
        if (filename) {
            this.pluginCache[filename] = plugin;
        }
        if (plugin.install) {
            plugin.install(this.less, this, functionRegistry || this.less.functions.functionRegistry);
        }
    }

    /**
     *