Add script to generate bundle sizes

[bjorntheart]

This PR fixes #25.

To generate the bundle size report, run npm run bundle:sizes.

I've attached the generated bundle-sizes.json file as an example.

The structure of the size objects are as follows:

/**
 * @typedef {Object} Size
 * @propety {number} raw - Unminified size in bytes
 * @property {string} size - Unminified size in KiB
 * @property {string} minified - Minified size in KiB
 * @property {string} brotli - Brotli compressed minified size in KiB
 * @property {string} gzip - Gzip compressed minified size in KiB
 */

To calculate the brotli and gzip compression sizes I added two packages brotli-size and gzip-size .

@argyleink looks like I found some time to finish this after all 😅

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