Refactor type generation scripts

[penalosa]

This is the first part of landing Types as a Service.

This is a relatively large PR, but it's pretty low risk. It mostly contains some refactoring of the type generation code (e.g. files like types/src/transforms/overrides/compiler.ts which have not changed in functionality). In large part this refactoring is to make it easier for a future PR (another part of #1959) to run this code in a Worker (for instance, changing import ... from "assert" to import ... from "node:assert" is in support of this goal, as well as the enhancing of the memory-based compiler host we use for running TypeScript).

This PR changes the output of the generated index.d.ts files to no longer declare things for which that's not necessary. This is semantically the same thing, and should have no impact on the usage of these types. We probably should've done it like this all along:

- declare interface SomeInterface {}
+ interface SomeInterface {}

- declare type SomeType =  {}
+ type SomeType = {}

However, this has resulted in some test changes—the behaviour is otherwise unchanged, and the output after this PR should be the same as the output before.

This PR doesn't cover:

• Generating types from internal modules. This means that e.g. cloudflare:sockets is still defined by hand, although this PR does add some additional mechanics to make this possible in future.
• The changes made in Implement types-as-a-service and check bundles with generated types #1959 to collation of comments from standards.
  • See [types] Refactor doc comment injection #2393
• Typechecking internal modules with generated types
• Implementing a worker to generate types
  • See [types] Generate @cloudflare/workers-types with a Worker #2405
• Using that worker when generating the @cloudflare/workers-types package in CI
  • See [types] Generate @cloudflare/workers-types with a Worker #2405

To aid reviewing, this link shows the diff between @cloudflare/workers-types@4.20240701.0 and the output of the type generation scripts run from this branch.

[andyjessop]

Looks great! One tiny comment, but the overall approach and implementation looks absolutely fine 👍