ESM

[Rich-Harris]

Converts config files to ESM and renames them from svelte.config.cjs to svelte.config.js, adds a message telling you to convert existing svelte.config.cjs files, and converts adapters to ESM.

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

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

[Rich-Harris]

Accidentally included #1321 commits. Oh well

[furudean]

(hooting and hollering)

[rchrdnsh]

yeehaw :-)

[Rich-Harris]

the build isn't going to succeed until the packages themselves are updated, so i'm going to merge and release