chore: simplify SSR

[Rich-Harris]

This simplifies SSR a bit — instead of having t_string and t_expression and t_statement, we just use ESTree nodes directly. One less wrapper object to deal with, one less type to understand, and it makes a lot of code more compact.

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 and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[changeset-bot]

⚠️ No Changeset found

Latest commit: 51ac6b8

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 has turned into a bit more of a general spring clean. I'm finding so much junk to throw out, feels great

[Rich-Harris]

Ideally we'd move this code around so it isn't in one 2,000+ line module, but I didn't do that for this PR so that it's possible to see what actually changed

[dummdidumm]

Nice cleanup, looks good to me. I'll defer to @trueadm for final approval because he built much of the initial server compiler and may have more insight.