chore: modularize server code

[Rich-Harris]

One thing that I've been wanting to do for a long time, but which #12530 really brought to a head, is to modularize certain parts of the codebase a lot more. It's just no good to have modules with multiple thousands of lines of code — it's impossible to create a good mental map of the codebase, and it leads to a messy and disorganized structure. It's much easier to repeatedly toggle between two side-by-side modules than it is to repeatedly scroll up and down a thousand lines, past reams of stuff that's irrelevant to my current task, when dealing with two interconnected functions.

There are other benefits:

• in the same way that it's very easy to find the runtime code for if blocks by doing Cmd-P if in VSCode, it's much easier to find the compiler code if I can do Cmd-P serIfB to find an IfBlock.js inside a server directory. Right now I tend to do Cmd-Shift-F and search for IfBlock( which is much slower (have to use keyboard and mouse together to avoid painful keyboard-only navigation, and can't use fuzzy matching)
• PRs are easier to review — less likely to hit GitHub's stupid 'we only show small diffs by default' limit, and the 'viewed' checkbox is more useful when you don't have a single file with hundreds of changed lines
• git history is more useful — you can more easily see which files were recently changed, etc

I'm starting with server transform visitors, because that's probably the least disruptive place to get the hang of this, but it's far from the only part of the codebase that needs this treatment.

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• 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

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