use 'browser' condition instead of 'node' to resolve ssr.mjs

[dominikg]

Describe the problem

currently, svelte exports ssr.mjs for '.' when the 'node' condition is set.
This changes onMount to be a noop and allows treeshaking bundlers to remove browser-only code and imports from ssr bundles.

but using the 'node' condition can lead to problems, eg if something deno only used 'deno' condition, it would not get ssr.mjs.

It's also not in alignment with esm-env that is going to be used by sveltekit.

Describe the proposed solution

use the browser condition, like esm-env by @benmccann
https://github.com/benmccann/esm-env

Alternatives considered

leave it as is, assuming the node condition is used for non-browser builds always

Importance

would make my life easier

[benmccann]

this has been addressed in the version-4 branch