Transform method option to put generated image disk cache in .cache folder

This works today to store output images into .cache/ and then copy those assets to the output folder post-build:

import fs from "node:fs";
import { eleventyImageTransformPlugin } from "@11ty/eleventy-img";

export default async function(eleventyConfig) {
	// Image optimization: https://www.11ty.dev/docs/plugins/image/#eleventy-transform
	eleventyConfig.addPlugin(eleventyImageTransformPlugin, {
		outputDir: ".cache/@11ty/img/",
		urlPath: "/img/built/",
	});

	eleventyConfig.on("eleventy.after", () => {
		fs.cpSync(".cache/@11ty/img/", "_site/img/built/", { recursive: true });
	});
}

Marginally related to https://github.com/11ty/eleventy/pull/3573

This is useful as some build servers persist .cache folder for you for free: https://www.11ty.dev/docs/deployment/#persisting-cache

Start with eleventyImageTransformPlugin and its handling of outputDir and urlPath; compare the current .cache-plus-copy workaround with the requested option. Review the issue discussion and related Eleventy pull request #3573 before defining acceptance criteria, then verify generated images are cached in .cache and available in the output folder without the post-build copy.