feat: transform Request, Response, and WebSocket classes to interfaces with var declarations

[andyjessop]

This PR is part of the drive for Node.js compatibility and focusses on ensuring 3 specific interfaces, Request, Response, and WebSocket, are compatible with @types/node.

The maintainers of @types/node considered a related use case when adding fetch types to Node, because it's relatively common for lib.dom and @types/node to be loaded together, which would cause similar conflicts to what we're seeing now.

Because of that, they use this pattern when defining a web type:

interface Response extends _Response {}
  var Response: typeof globalThis extends {
      onmessage: any;
      Response: infer T;
  } ? T
      : typeof import("undici-types").Response;

This defers to the global Response type if it's available, falling back to undici otherwise.

In order to get this to work for us, we need to declare declare var onmessage: never, then Response will be our Response.

Next, we need to get these three interfaces onto globalThis. In order to do that, our class delcarations need to be converted to var declarations. And in order to get the same type of behaviour (e.g. in return types), we need a corresponding interface. For example:

interface Request<
  CfHostMetadata = unknown,
  Cf = CfProperties<CfHostMetadata>,
> extends Body {
  ...
}
declare var Request: {
  prototype: Request;
  new(input: RequestInfo | URL, init?: RequestInit): Request;
};

Implementation

This PR creates two transformers that will transform our types in the worker:

1. createAddOnMessageDeclarationTransformer - adds the onmessage declaration.
2. createClassToInterfaceTransformer - converts our classes to an interface+var declaration pair.

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[penalosa]

This looks good to me! It would be great to see the diff of types generated with this turned on vs with this turned off, to verify the behaviour is correct

[andyjessop]

@penalosa This gist shows a comparison of the new vs old generated types: https://gist.github.com/andyjessop/ca8456d22b2abb44542143d55c196040