feat(vercel): ESM support and Node.js 16

[JuanM04]

Fixes #4782

This PR adds support for ESM deployments to Vercel by adding a package.json with { "type": "module" } (AWS post about this). It also allows the user to select which Node.js version they want to use (it can be 16.x (default), 14.x or 12.x). It also allows deploying to Node.js 16, which can be configure inside the Vercel Project's settings

Please don't delete this checklist! 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
• 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 pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpx changeset and following the prompts. All changesets should be patch until SvelteKit 1.0

[changeset-bot]

🦋 Changeset detected

Latest commit: 419dc8d

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/adapter-vercel	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[Rich-Harris]

Thanks — can we auto-detect the node version? The same version is used during build as in the deployment, so we should be able to get rid of the option altogether and Do The Right Thing based on process.version (including throwing an error if it's 12)

[JuanM04]

@Rich-Harris Done!

[Rich-Harris]

thank you!