build(server-renderer): Add ESM as a build target

[Igloczek]

Scope of changes:
Add ESM as a build target for @vue/server-renderer

Reasoning:
I'm trying to fix Vue compatibility with Astro, which is build on top of Snowpack, so build tool that doesn't like non-ESM sources, and that causes issues with code duplication and breaking solutions based on being singletons, like currentRenderingInstance, that leads to errors while trying to render nested components.

Exposing this package as ESM is probably the simplest way to solve that problem, and from what I see some changes in this area was already made, for example ee4cbae, so hope it's inline with the vision.

[yyx990803]

Hmm, I realized this alone probably isn't enough since @vue/server-renderer depends on @vue/compiler-ssr which is currently cjs-only.

[yyx990803]

Oh I see the original issue is about duplicated Vue runtime so I guess leaving @vue/compiler-ssr as-is should be fine.

[Igloczek]

From what I tested locally, it works with just this one in ESM, since the ssrUtils was the main problem, so unless there is some singleton-wannabe stuff in @vue/compiler-ssr, we should be fine.