[chore] dev mode refactor

[Rich-Harris]

WIP. follow-up to #3129. Using Vite's watcher means we can push more logic into the plugin. Also, Watcher extends EventEmitter but we're not actually emitting any events, so we can make all that stuff way simpler

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]

⚠️ No Changeset found

Latest commit: a802fc7

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

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

[Rich-Harris]

This will fix #2793

[Rich-Harris]

Bit weirded out by these deployment failures. Investigating

[Rich-Harris]

deploys all passing — issue turned out to be netlify being a bit overzealous with the build cache