Test-treeshake: use terser for minification

[marcofugaro]

Related issue: -

Description

We were using rollup-plugin-filesize to show the minified size in the test-treeshake script. However the minified sized were skewed because it was calculating them internally.

I've updated the script to use terser to compute the minified size. Terser is what most people use nowadays to minify, so it's more accurate.

The script now produces index.bundle.min.js and an unminified index.bundle.js for human inspection, this is the new output:

[mrdoob]

Thanks!