fix: support polyfill import paths containing an escaping char (e.g. '\')

[Tal500]

Description

When the additionalPolyfill property of plugin-legacy containing polyfill paths with escaping characters (e.g. '\'), the JS fails to compile because plugin-legacy doesn't escape them.
This PR add the escaping characters for this, simply by the trick of JSON.stringify().
This is a breaking change, since old code (e.g. my code) may use a hack to pass an escaped strings to additionalPolyfill, so I suggest to merge it now for Vite 4.

An example of the issue is when you want to include an absolute path of files in Windows (which uses '\' for directory paths), this was my previous sample with my own escaping workaround:

import { dirname, resolve } from 'path';
import legacy from '@vitejs/plugin-legacy';
import type { UserConfig } from 'vite';

const __dirname = dirname(fileURLToPath(import.meta.url));
const localPath = (path: string) => JSON.stringify(resolve(__dirname, path)).slice(1, -1);

const config: UserConfig = {
	plugins: [
		legacy({
			additionalLegacyPolyfills: [
				'custom-event-polyfill',
				'core-js/modules/es.promise.js',
				'whatwg-fetch',
				// ...,
				localPath(...),
				// ...,
			],
		}),
	]
};

export default config;

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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