_applyOnMultiCompiler(compiler, handler) {
		const states = compiler.compilers.map(
			() => /** @type {[number, ...string[]]} */ ([0])
		);
		for (const [idx, item] of compiler.compilers.entries()) {
			new ProgressPlugin((p, msg, ...args) => {
				states[idx] = [p, msg, ...args];
				let sum = 0;
				for (const [p] of states) sum += p;
				handler(sum / states.length, `[${idx}] ${msg}`, ...args);
			}).apply(item);
		}
	}
@param {MultiCompiler} compiler webpack multi-compiler
@param {HandlerFunction} handler function that executes for every progress step
@returns {void}