chore: remove UMD build

[benmccann]

This file is not referred to in the package.json. Removing it will make the package about 30% smaller

[antfu]

We still mention it in the CDN usage: https://github.com/Rich-Harris/magic-string#installation
I don't think we should remove it

[benmccann]

Ah, you're right. Still I wonder if we can't remove it the next time we cut a breaking release which would be the next minor version since we're still pre-1.0 (i.e. 0.30.0). I can't imagine a usecase where you'd use it via a CDN. I expect the vast majority of users use it via npm. Any users who do use it via a CDN could change to build their bundles via npm before deploying.

The reason I'm interested in reducing package size is that learn.svelte.dev loads the Svelte compiler's dependencies from npm into the browser. The smaller those dependencies are the faster the tutorial will load. With Svelte v4 we're really slimming down the dependencies - I've already gotten it from 61 down to 16. All of Rich's other package are quite small as they've been updated to distribute only the ESM source. magic-string is the largest of his packages because it distributes three versions and sourcemaps for all of them where the sourcemaps are 2x the size of the code itself.