Replace magic-string in SSR bundles with a bare bones replacement

The `magic-string` package is 19.5kb minified, and we don't actually need all those features. For example, it can produce sourcemaps, but SSR bundles work with pre-compiled JS modules, so there's no use case for sourcemap generation. In the SSR bundle, `magic-string` is only used in HTML manipulation, where sourcemaps aren't supported.