[feat] export svelte/ssr in node

[bluwy]

Credit to @benmccann for the idea. This prevents the need for bundlers to bundle external dependencies that imports svelte in order to resolve them to svelte/ssr.

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with [feat], [fix], [chore], or [docs].
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with npm test and lint the project with npm run lint

[benmccann]

lgtm. It looks like vite-plugin-svelte is the only thing that uses the svelte/ssr export. So if we cut a new release of the plugin that stops using it we could probably remove the svelte/ssr export

[Conduitry]

Removing svelte/ssr would technically be a breaking change. We don't know what other third-party tooling out there might have started using it. That said, I do think the risk is fairly low, and the it's a question then of how much of a stickler we want to be for semver.

[Conduitry]

This export map entry is now included in 3.43.0, thanks!